Sorry Mike, got to disagree with you on that one, been there, ain't going to happen. The problem is the Z31 input shaft is about 3 inches longer than the 280ZXT input shaft, but you can't swap the 280ZXT shaft in to the Z31 box as the 280ZXT has 24 teeth on its lay gear whereas the Z31 has 23T. I tried for ages to source a different input shaft of the right lenght with 23T gear which in theory exist but with a different clutch spline pattern (so hassle getting a matching clutch), but I just couldn't get hold of one for love nor money. The other suggestion was to do a cut'n'shut on the Z31 input shaft to get it to the right length, but that's hardly adviseable given the desire for it to handle more power. Final nail in the coffin for the Nissan T5s is that they're the 'Non-World-Class' version (the Cossie box is the 'World Class' version), that means they're nowhere near as strong as the WC boxes, my research even turned up people swapping them out of their 280ZXs to put the Nissan box in as that was stronger.
I went down the 200SX gearbox swap. You take the front half/bellhousing of your original Zed 5 speed, do a bit of machining on that, then it's a direct fit on the the 200SX gearbox. Some fiddling needs to be done to the rear mount (I'm about a lifetime away form worrying about that admittedly!) but you have a much stronger box than the original Zed one, that can be rebuilt to handle lots more power - check out
http://www.phattransmissions.com/PhatTrans/transmissionBuilds.html
Fitting a WC T5 to the 280ZXT T5 Bellhousing is also a good alternative. The bellhousing apparently needs some machining to fit the WC input shaft retainer but that's pretty straightforward. You can get a WC T5 rebuilt to handle mucho power to:
http://www.gearboxman.co.uk/infot5.html
Cheers,
Rob