Today I rode in an 87GT with a stock Getrag, but it had a Stage 2 4-puck clutch in it. It also had a 3800 supercharged engine. The transmission was holding it very well under some pretty hard acceleration.
To install the 3.4 into a 2.8 car, the only thing that needs done is to drill and tap to relocate the starter to the left side of the engine. If it's an automatic, you will need to grind off a small portion of the original starter boss on the right side. If it's a manual, nothing else needs done. Just transfer everything from the 2.8 to the 3.4.
For a 3800, you need a new OBD 2 computer and have it reprogrammed, new wiring harness, engine and transmission mounts and a flywheel. Possibly a new fuel pump, depending upon if you're doing an NA or SC and a new exhaust system. Probably other stuff, but those are the basics.
Then with the 3800's extra Go, you'll probably find you need some extra Whoa, so a brake upgrade would be in order. Probably the Grand Am upgrade would be the cheapest.
If you're upgrading from a 4 cylinder, the same applies, but for the 3.4, you would need a new V6 OBD 1 computer from a V6 and an engine compartment harness. Fiero ECM's are specific to the engine/transmission combo except that a V6 manual ECM of any year works for all manual V6 2.8 or 3.4's.