Not a lesson learned... but relatively simple math.
Sticking with the stock 15" sizes for now:
88 uses 15x6 wheel with +37mm. I mention this because Archie makes a bracket to use the LeBaron brakes on an 88. However, since the 88 has a separate hub, there is no need to make a hub, and thus, except for the difference in thickness of between the LeBaron rotor and the 88 Fiero rotor, the stock wheel would remain in the same relative position.
84-87 uses 15x7 wheel with +30 offset. To use almost every substitute brake package (e.g. Beretta/Grand Am, LeBarn, etc.) the front rotor must be parted off from the remainder to make a hub. The new rotor sits against the face of the newly made hub. These rotors are typically 9-10mm. Thus, if you want to maintain the same spacing as stock, you would need to use a +39-40mm offset wheel.