Why do you have to register it as a Fiero? The truth is, here in California nobody cares, and the DMV never checks, unless you ask them to. They wouldnt know what they were looking at anyway!
I have an '02 Tahoe that has been registered as a commercial vehicle since I bought it new at the dealership. I never thought about it before, even though I had commercial plates on it until my wife asked me why my registration was so high on it (3X what she pays on her '02 Suburban).
I went to the DMV and asked why my car was a commercial vehicle and they told me because it has a "bed" and can carry loads. I said it was an SUV and they said it didnt matter, and I would have to have a referee look at it (lol). Needless to say, its now registered as a passenger vehicle, no commercial fees.
My point is, the DMV couldn't care less what you wanna register your car as.
Lets say you wanna have a 3800SC in your Fiero kitcar. Fiero's never came with a 3800, so thats gonna be a problem with smog, but not the DMV.
To get around the smog, find out what year and model vehicle the engine you're using came out of, and register it as that. The easiest way to do it is to have a VIN from the same model/year vehicle (salvage or not).
Nobody (DMV, CHP, Insurance, etc) is gonna question the vehicle if it displays the proper VIN, and nobody will ever know if its a custom chassis or a donor vehicle. If the engine you're using comes out of a '02 Pontiac Grand Am, then your car (as far as they are concerned) better be built on what you call a modified '02 Pontiac Grand Am, and have a VIN to match. It can be a tube chassis, it only needs to have the VIN!
In California, it really is just that simple.