1984 Pontiac Fiero cooling system

You listed a 3.5L engine which was never available in a Fiero. Are you doing some kind of transplant or do you have the standard 2.5L?

How are you determining the engine is overheating? Engines don't typically get hot fast enough to overheat in just five minutes. Are you seeing bubbling in the coolant reservoir? That is more likely a sign of a leaking cylinder head gasket. The clue is there won't be any steam associated with that bubbling.

If the coolant really is getting too hot so quickly, an inoperative radiator fan is the most likely suspect. The size of the original radiator is sufficient. Going to a bigger one doesn't address the cause of overheating. Feel the radiator hoses. If they are cool, the coolant isn't flowing to the radiator to give up its heat.