Welcome to the Forum.
Personally, I think the 3.8 is a better choice for the Fiero, but rock on with the 4.9 monster
Air intakes helped my mileage, but I have a different motor. When I first joined this site, I read this thread about a cowl induction, ram air mod that is low budget, and works great
Check it out HRW Mod
As far as the other stuff goes, just familiarize yourself with the specialized Forums and you'll find someone who knows exactly what you're looking for.