I have done several of these upgrades (about 6 IIsi's, and 1 Quadra 700), and
the best thing to use would be some sort of "heat sink compound". If possible,
you should look for the silicon-free stuff. There's a comany who makes the stuff
called Tech Spray, their address is: P.O. Box 949, Amarillo, TX 79105.

You should be wary in using most kinds of tape; and definately don't use duct
tape  that stuff is for ducts...

When using the heat sink glue or compound, only use enough to fill the small
space between the heat sink and the CPU.

Mario Murphy