Welcome to the forum, Toysport.
The answers to your inquiries can be found deep within the bowels of the Chevy forum. In other words, bro, use the search function.
However, to steer you in the right direction, you can swap any year of V8 into your Z so long as it is the same year or newer as your 280Z. You must comply with CA smog laws, so this rule is manditory. For easy reference on how to go about the swap, buy the JTR manual from http://www.jagsthatrun.com and read it over a few times. That will set the stage for a comprehensive outline of the necessary steps involved.
As for the driveshaft, the JTR manual will cover that, and as for the diff, the 280Z comes with an R200 that is plenty strong to cover most V8s. Again, that is covered here in the forum and the JTR book. The wiring harness depends on what year of 350 you'll be using, whether carbed or EFI. If you go EFI, use EVERYTHING from the donor vehicle.
Search the forums for nuggets of gold and download them.
Davy