This Camaro has a V6 that blew, I tried to replace it with a used engine, now it rattles
as if it would fall apart. Some mechanics I've taken it to say it's probably a bent flywheel (aka. flexplate) Any ideas?

Quite a few of the later engines require a weighted flywheel to balance the engine. Called external balance flywheel. Check with local parts place to determine if the engine year and model you installed requires this. tom