It's possible, in fact here's a 2JZ-GTE in an '88 or so z31, click on the engine link.
http://www.az-zbum.com/modification.engine.swap.shtml
Toyota Supra
Yes it will
If they both have the same engine.
yes it can.
A 1985 Nissan 300ZX has a 3.0L SOHC V6 engine ... therefore, 6 cylinders.
3.0 Nissan motor 3.0 Nissan motor
Yes
You will have to remove the engine.
The alternator when the engine is on, and the battery when the engine is off.
yes with heavy modification
Yes, it's possible.
Nissan 300zx not even close with Toyota supra