Umm... Fieros don't have frames...
I am not a Fiero expert, but seem to remember something about the body panels being plastic, which would mean that there is a steel frame under the plastic panels.
I honestly don't see why you would do this, seems like you could build a better motor for less, but to each his own.
The four and six cylinder engines available in the Fiero were garbage. I've actually received inquiries from Fiero owners interested in transplanting in a Isuzu 1.6 liter turbo DOHC once they figure out that the four cylinder Fiero came with an Isuzu transmission. But 160 HP seems to be less appealing than the V8 swaps.