www.z31.com www.z31.net
I can give you links to many Z sites (forums, parts distributors, JDM sites, etc.). Read up, read some more, then let me know if you can't find something. I'd like to help if it's for the sake of a Z.
The motors are very very similar. The turbo Z31's come with a stock T3 turbo. The older Z's pull very hard when boost kicks in. You do not need twin turbos on that car. Stick with the stock engine and build it to turbo spec (don't forget the oil squirters) or buy a used Z31 turbo motor and swap. While the engine is out, replace all old and worn hoses. They are cheap and this is the best time to do it. I would replace anything else that looks old or nasty. You get 25% off OE parts from Courtesy Nissan if you tell them you are a member of www.twinturbo.net.
If you want an intercooler, you'll have to go custom as the older Z's did not have one. I suggest a brand new custom fit core or a stock Supra TT core. Don't go the junkyard route and use a Saab or Starion IC. If this is gonna be a track-only car, I wouldn't even bother with an intercooler.
No, it should not. Stay VG30.
The only 6speed Z is the 350Z. I can't imagine any newer trannies will bolt up easily to the Z31 motor, but then again I am not sure. I wouldn't worry so much about this conversion. A 5 speed will be fine, and there are plenty of clutch and flywheel upgrades. The automatic is fun too and is very quick on the turbo Z's.
You need to order a Motorsport catalog for those parts. The older Z's are very prone to rust. You may want to go to a Z shop or good body shop and find out what kind of shape it is really in. Garages that restore old Z's replace pretty much the entire car with new metal.