The Fiero - and all the common popular swaps - is water-cooled so heat is not a problem in most occurences. The engine obtains operating temperature just like any other engine. The only time heat would be a problem is if you have some sort of problem with the cooling system itself, or it is inadequate for your current swap. For most V6 swaps the OEM cooling setup is fine. For some larger engines that not only take up room in the engine compartment but are also overall larger, normal cooling setups are inadequate.