Swapping a GTR motor into the Z is stupid. could it be done, yes, anything's possible with deep enough pockets. You won't be able to reuse the tranny though, since the GTR's is rear mounted and I don't want to think aobut the reengineeirng that would be needed to make that happen.
Cheap and reliable motor swap for more power: 370z w/ an LSxx motor and tranny dropped in.
/thread.