Don't know much about the Fiero, but they have never been impressive on track. ITB cars use the Iron Duke which is pretty anemic in IT trim, pushing a heavy chassis.
Mustangs are interesting...the After market support is there for the V8 stuff...read the other letters on the Ford site. Very little other than bushings, sway bars and wheels are available for IT cars. Brake pads are difficult to find if other than parts store stuff. Engines are generally anemic, the 8 plug included. The interesting part of a Mustang build is the considerable number of years that you have to choose from in parts...all on the same line.
The car should not be lowered excessively due to the Roll center falling off the face of the earth. The rear suspension sucks, since it will go into bind with roll and give snap overstear when the roll resistance of the rear goes wild from the rubber bushing bind. Gearing is difficult but there are bizzare possiblilties with machining and cleverness. Transmission ratios suck, and the car is an aerodynamic brick to be used by others as a vehicle to draft...if it went fast enough.
Some changes in the rear will reduce or eliminate the problem source, but it requires fabrication of parts, carefull reading of rules (legal), and money.
Struts for the front are DIFFICULT...Koni can be made to work after you re engineer the existing shock. Don't know of any inserts available for this chassis. Street units are not made for the necessary heavy spring rates.
Generally, the car is good fun, easy to drive after the rear problem is settled, great learning tool, safe from the mass perspective, very reliable, and the parts are terribly cheap. Spares are available at almost any Junk Yard. With judicious part selection minimum weight can be made with this car, but I doubt it would be with the newer chassis. They got heavier as time went by. We went with the earlier year chassis and a carb, building it from the ground up. Buying completed cars makes sense depending on your skill level and what the intended final result will be. Want to have the best example, build it from scratch. (That comment is intended for Mustang construction only.)
We built two of them. Would I do it again? Probably not, since I want to be competitive. I really prefer Rear wheel Drive for the shear fun of control, and balance. Currently considering ITS for that reason.
Good luck with your decision. :
Good racing.
Bill