It depends on what heads your donor 3.1 engines have. Basically the iron head 3.1 motors have the same block and head chamber size as the 2.8 iron head motors, the only real differences between the two are the pistons and crank, and the flywheel/flexplate on all 3.1s is neutrally balanced whereas the Fiero didn't get neutral balance flywheel/flexplates until the '88 year. To do this swap, just swap the crank and pistons into the Fiero block and get a neutrally balanced flywheel/flexplate for the '85 GT.
If the heads are aluminum it's a whole different ballgame. Nothing above the block will interchange between the two.The pistons are matched to the head and if used with the Fiero heads will yield a compression ration down in the sevens. About the only thing useful out of the aluminum head motors is the crank and rods, everything else will need to be bought. If you want to use the whole motor it's a real engine swap with ECM replacement and the whole nine yards.