The best engine for any of the 3rd generation F-bodies was the 350 TPI, so if you want to drop in the best factory motor, that'd be it. If the car is an auto, you'll be fine, but if it is the 5 speed, you might want to change to a different trans. As I understand it, the 5 speeds never came with a 350 because they could not withstand the power in the long run.
Since the TPIs are computer controlled, make sure you get the computer for your engine. Earlier TPIs were MAF, and the later ones ('91-'92 I believe) were Speed Density.
I'm not sure if you can put a traditional Pontiac motor in, especially if your emissions laws are strict. Go over and register on thirdgen.org, as it is the best site out there for this generation of F-bodies. People there would know. I've taken a real shine to the 350 TPI, so I feel no desire at all to go 400 Poncho.
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