Document:

<PAGE>

                                                                 EXHIBIT 10.37.3

                              AMENDMENT NO. 6 TO
                              ------------------
                        PURCHASE AND SALE AGREEMENT AND
                        -------------------------------
                           JOINT ESCROW INSTRUCTIONS
                           -------------------------
                           AND SIDE LETTER AGREEMENT
                           -------------------------

          THIS AMENDMENT NO. 6 TO PURCHASE AND SALE AGREEMENT AND JOINT ESCROW
INSTRUCTIONS AND SIDE LETTER AGREEMENT (this "Amendment") is made and entered
                                              ---------
into and effective as of March 6, 2000, by and between KAISER VENTURES INC., a
Delaware corporation and KAISER STEEL LAND DEVELOPMENT INC., a Delaware
corporation (collectively, "Seller"), and ONTARIO VENTURES I, LLC, a Delaware
                            ------
limited liability company ("Buyer").
                            -----

                                   RECITALS
                                   --------

          A.   WHEREAS, Buyer and Seller have entered into that certain Purchase
and Sale Agreement and Joint Escrow Instructions dated as of October 19, 1999 as
amended by that certain Amendment No. 1 to Purchase and Sale Agreement and Joint
Escrow Instructions and Side Letter Agreement dated as of November 2, 1999, by
that certain Amendment No. 2 to Purchase and Sale Agreement and Joint Escrow
Instructions and Side Letter Agreement dated as of November 12, 1999, by that
certain Amendment No. 3 to Purchase and Sale Agreement and Joint Escrow
Instructions and Side Letter Agreement dated as of November 19, 1999, by that
certain Amendment No. 4 to Purchase and Sale Agreement and Joint Escrow
Instructions and Side Letter Agreement dated as of November 30, 1999, and by
that certain Amendment No. 5 to Purchase and Sale Agreement and Joint Escrow
Instructions And Side Letter Agreement dated as of February 4, 2000
(collectively, the "Purchase Agreement"); and
                    ------------------

          B.   WHEREAS, pursuant to the Purchase Agreement, Buyer and Seller
have entered into that certain Side Letter Agreement dated as of October 19,
1999 which side letter was amended by the amendments to the Purchase Agreement
described in Recital A above (as amended, the "Side Letter Agreement"), the form
                                               ---------------------
of which was attached to the Purchase Agreement as Exhibit DD; and
                                                   ----------

          C.   WHEREAS, Buyer and Seller desire to again amend the Side Letter
Agreement and the Purchase Agreement, as more particularly set forth herein.

          NOW, THEREFORE, in consideration of the mutual covenants and
agreements herein contained and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Buyer and Seller
hereby agree as follows:

                                   AGREEMENT
                                   ---------

     1.   Amendments. The Purchase Agreement and Side Letter Agreement are
          ----------
hereby amended as follows:

                                       1
<PAGE>

          A.   The "Contingency Date" (as defined in Section 1.11 of the
Purchase Agreement) shall be extended further to March 10, 2000.

          B.   The date by which Buyer and Seller shall negotiate in reasonable
good faith to finalize the remaining provisions of Exhibits E, G, K, L, M, M-I,
Q, Y and CC and attach them to the Purchase Agreement shall be further extended
to March 10, 2000.  During the time between the date hereof and March 10, 2000,
the Parties shall diligently use reasonable good faith to finalize such
Exhibits.  In the event that all of such Exhibits are not so agreed upon by such
date, then either Buyer or Seller may terminate the Purchase Agreement, in which
event the provisions of Section 4.7 thereto shall apply except that any portion
of the Deposit theretofore released to Seller shall not be refunded to Buyer.

          C.   Section 4.9 of the Purchase Agreement is hereby amended by
extending the date by which Buyer and Seller shall negotiate in good faith to
finalize the IT Contract and performance bond therefor or each of the Insurance
Policies so that they are acceptable to Seller to March 10, 2000.  In the event
that all of such Insurance Policies, the IT Contract and the performance bond
therefor are not so agreed upon by such date, then either Buyer or Seller may
terminate the Purchase Agreement, in which event the provisions of Section 4.7
thereto shall apply except that any portion of the Deposit theretofore released
to Seller shall not be refundable to Buyer.

     2.   No Other Amendments. Except as expressly amended hereby, the Purchase
          -------------------
Agreement and the Side Letter Agreement are unchanged and in full force and
effect.

     3.   Counterparts. This Amendment may be executed in counterparts, each of
          ------------
which shall be an original and all of which taken together shall constitute the
same instrument.

          IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the dates written below.

                                       2
<PAGE>

Buyer:                       ONTARIO VENTURES I, LLC, a Delaware limited
-----                           liability company

                                By:  LandBank Environmental Properties, LLC, a
                                     Delaware limited liability company, its
                                     Manager

                                     By:  LandBank, Inc., a Delaware
                                          corporation, its managing member

                                          By:  /s/ Stuart L. Miner
                                               --------------------------------
Date Executed:   03/06/00                 Its: Vice President
               -------------                   --------------------------------

Seller:                         KAISER VENTURES INC., a Delaware corporation
------

                                By:  /s/ Terry L. Cook
                                     ------------------------------------------
Date Executed:   03/06/00       Its: Executive Vice President & General Counsel
               -------------         ------------------------------------------

                                KAISER STEEL LAND DEVELOPMENT INC., a Delaware
                                corporation

                                By:  /s/ Terry L. Cook
                                     ------------------------------------------
Date Executed:   03/06/00       Its: Vice President
               ------------          ------------------------------------------

RECEIVED AND ACCEPTED THIS ____ DAY OF __________________, 2000.

ESCROW HOLDER:

CHICAGO TITLE INSURANCE COMPANY

By:_________________________
Its:________________________

                                       3<PAGE>

                                                                 EXHIBIT 10.37.4

                              AMENDMENT NO. 7 TO
                              ------------------
                        PURCHASE AND SALE AGREEMENT AND
                        -------------------------------
                           JOINT ESCROW INSTRUCTIONS
                           -------------------------
                           AND SIDE LETTER AGREEMENT
                           -------------------------

          THIS AMENDMENT NO. 7 TO PURCHASE AND SALE AGREEMENT AND JOINT ESCROW
INSTRUCTIONS AND SIDE LETTER AGREEMENT (this "Amendment") is made and entered
                                              ---------
into and effective as of March 10, 2000, by and between KAISER VENTURES INC., a
Delaware corporation and KAISER STEEL LAND DEVELOPMENT INC., a Delaware
corporation (collectively, "Seller"), and ONTARIO VENTURES I, LLC, a Delaware
                            ------
limited liability company ("Buyer").
                            -----

                                   RECITALS
                                   --------

          A.   WHEREAS, Buyer and Seller have entered into that certain Purchase
and Sale Agreement and Joint Escrow Instructions dated as of October 19, 1999 as
amended by that certain Amendment No. 1 to Purchase and Sale Agreement and Joint
Escrow Instructions and Side Letter Agreement dated as of November 2, 1999,  by
that certain Amendment No. 2 to Purchase and Sale Agreement and Joint Escrow
Instructions and Side Letter Agreement dated as of November 12, 1999, by that
certain Amendment No. 3 to Purchase and Sale Agreement and Joint Escrow
Instructions and Side Letter Agreement dated as of November 19, 1999, by that
certain Amendment No. 4 to Purchase and Sale Agreement and Joint Escrow
Instructions and Side Letter Agreement dated as of November 30, 1999, by that
certain Amendment No. 5 to Purchase and Sale Agreement and Joint Escrow
Instructions And Side Letter Agreement dated as of February 4, 2000, and by that
certain Amendment No. 6 to Purchase and Sale Agreement and Joint Escrow
Instructions And Side Letter Agreement dated as of March 6, 2000 (collectively,
the "Purchase Agreement"); and
     ------------------

          B.   WHEREAS, pursuant to the Purchase Agreement, Buyer and Seller
have entered into that certain Side Letter Agreement dated as of October 19,
1999 which side letter was amended by the amendments to the Purchase Agreement
described in Recital A above (as amended, the "Side Letter Agreement"), the form
                                               ---------------------
of which was attached to the Purchase Agreement as Exhibit DD; and
                                                   ----------

          C.   WHEREAS, Buyer and Seller desire to again amend the Side Letter
Agreement and the Purchase Agreement, as more particularly set forth herein.

          NOW, THEREFORE, in consideration of the mutual covenants and
agreements herein contained and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Buyer and Seller
hereby agree as follows:
<PAGE>

                                   AGREEMENT
                                   ---------

     1.   Amendments. The Purchase Agreement and Side Letter Agreement are
          ----------
hereby amended as follows:

          A.   The "Contingency Date" (as defined in Section 1.11 of the
                    ----------------
Purchase Agreement) shall be extended further to March 13, 2000.

          B.   The date by which Buyer and Seller shall negotiate in reasonable
good faith to finalize the remaining provisions of Exhibits E, G, K, L, M, M-I,
Q, Y and CC and attach them to the Purchase Agreement shall be further extended
to March 13, 2000.  During the time between the date hereof and March 13, 2000,
the Parties shall diligently use reasonable good faith to finalize such
Exhibits.  In the event that all of such Exhibits are not so agreed upon by such
date, then either Buyer or Seller may terminate the Purchase Agreement, in which
event the provisions of Section 4.7 thereto shall apply except that any portion
of the Deposit theretofore released to Seller shall not be refunded to Buyer.

          C.   Section 4.9 of the Purchase Agreement is hereby amended by
extending the date by which Buyer and Seller shall negotiate in good faith to
finalize the IT Contract and performance bond therefor or each of the Insurance
Policies so that they are acceptable to Seller to March 13, 2000.  In the event
that all of such Insurance Policies, the IT Contract and the performance bond
therefor are not so agreed upon by such date, then either Buyer or Seller may
terminate the Purchase Agreement, in which event the provisions of Section 4.7
thereto shall apply except that any portion of the Deposit theretofore released
to Seller shall not be refundable to Buyer.

     2.   No Other Amendments. Except as expressly amended hereby, the Purchase
          -------------------
Agreement and the Side Letter Agreement are unchanged and in full force and
effect.

     3.   Counterparts. This Amendment may be executed in counterparts, each of
          ------------
which shall be an original and all of which taken together shall constitute the
same instrument.

                           [Signatures on next page]

                                       2
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the dates written below.

Buyer:                      ONTARIO VENTURES I, LLC, a Delaware limited
-----                           liability company

                                By:  LandBank Environmental Properties, LLC, a
                                     Delaware limited liability company, its
                                     Manager

                                     By:  LandBank, Inc., a Delaware
                                          corporation, its managing member

                                          By:  /s/ Timi Anyon Hallem
                                               -----------------------------
Date Executed: __________                 Its: Agent
                                               -----------------------------

Seller:                         KAISER VENTURES INC., a Delaware corporation
------

                                By:  /s/ Terry L. Cook
                                     ------------------------------------------
Date Executed:  03/10/00        Its: Executive Vice President & General Counsel
               ----------            ------------------------------------------

                                KAISER STEEL LAND DEVELOPMENT INC., a Delaware
                                corporation

                                By:  /s/ Terry L. Cook
                                     ------------------------------------------
Date Executed:  03/10/00        Its: Vice President
               ----------            ------------------------------------------

RECEIVED AND ACCEPTED THIS ____ DAY OF _________________, 2000.

ESCROW HOLDER:

CHICAGO TITLE INSURANCE COMPANY

By:______________________
Its:_____________________

                                       3

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