If you REALLY wanted to do it, get a plasme cutter!!!, and cut the whole floor pan, with all sispension pickups and mounts, out of the z31, do the same with the zx, and just weld the z31 back half inner floor pan.
Thete will be a few gaps, Sikaflex will fix these.
The upper strut top mounts may need playes welded onto the zx, just cut these out of the 31 and weld them in.
Try to measure from front crossmember to rear points to ,ake sure you get the new grafted rear floor pan as square as possible.
Make sure its all done on an already registerd zx, that way you wont need to get it engineered.
Back in old days when fronts of one car would be welded onto back of anotjer car, ie re-birthing, we would use black schutz a thick tar like coating on the underside, to hide joins. For the interior, just use heaps of glue to glue carpet to floor pan, that way, no-one can lift carpet to check for any dodgy business.
I wont tell, if ypu wont tell, OK?
