[D13]

Assuming the 89 is a square body and the 94 is a GMT400:

1. no.
2. no, except the compressor if swapping the engine.
3. Yes, with some thought. You may have to swap sensors to match the inputs the earlier ECM was looking for, and the fan to radiator shroud will have to be looked at, and the exhaust manifolds 'might' need to be swapped. PS hoses and such 'should' be OK, unless the crew has hydroboost. Cruise set up is differnet, and a bazillion little things like wire lengths, starter / flexplate/trans, etc. But the basic motor should swap.