Locate crossmember with rack mounts forward under the frame, centering it with your marks on the frame. With chassis sitting at anticipated ride height and rake, check crossmember for level forward and aft as will as side to side. If necessary, shim with weldable steel material until everything is right. Now measure lower control arm holes, making sure they are equal distance from each side. Double and triple check all measurements, then tack weld in place.
Check measurements once more then weld in place, alternation sides to prevent overheating frame.
Set this assembly on your frame centering it with your axle centerline marks as well as side to side. The spring towers mount with the notched side to the back. Check measurements then tack weld in place. Next tack weld the gussets in place, angling them in to meet the lower crossmember. Short gussets in the rear, long ones in the front.