the pistons and rods from a ZXT are the same as in your motor. The head is what you need to change, and that needs to be changed to a P90. The P90 (81 and 82 ZXT's) has the square exhaust ports and most importantly a high quelch chamber. You might be able to run a N42 head but i would think it would dampen the power and limit how high you could go on boost.
another difference in the 77 and a ZXT is the block. you have an N42 block, the turbos came with a F54, which is supposed to be stronger it is the same siamesed cast block as the N42 it just has flow holes for coolant.....so i dont know if it is really stronger or not.