It depends on how far you want to go- Yes, I know you said ultimate but that means different things to different people; To stay with the original Fiero 60* V6 family, the top engine would be the 3900 w/ 240 hp/ 240 Ft/Lb....And a few lbs lighter....Or you could go with an LS/LT V8- Slightly more weight but much more power- 300(LS4) up to 505(LS7) and that is before engine mods and avoiding supercharging. Of course, then you have to get a transmission that will survive the torque of the new engine.
The chassis is rigid enough, but a little seam welding would make it even better. And the suspension needs some tuning- My personal opinion is that an early front end with an 88 rear would be best- The early front is not that bad and is more robust with easily accessible bearings and spindle. The 88 rear works properly- better ride and handling...Even if you shortened the toe link on the early suspension it doesn't have the ride and bump absorption of the 88- plus the 88 cradle is solid mounted which acts as reinforcement for both the cradle and the chassis.
We have a wide selection of brakes to chose from- I would suggest not going too big- rather keep the weight down- both unsprung and rotational. And shocks....I sure wish we could retrofit a set of those magnetorheological shocks they have on new cars.....
I have had an idea percolating in my brain for the past decade- A Fiero with the LS7- basically using as much "production" style parts, etc; You can fit C5 Vette wheels under the stock fenders, but slightly lower profile tires.....Call it either the "Group 7 Fiero" or the "Hulki GT" and then have someone who is really good drive it around a course that one of the mags test at- VIR, Laguna Seca, etc...and lay a really good time down...If the testing was publicized on Top Gear, etc or in one the magazines, it would certainly help to cut back on grief I get from almost everyone about Fieros......"They catch fire easily!" "typical GM crap!"
I almost forgot; The GTU racecars were like Nascar "Stock cars" or any other racecar chassis with body panels- Not a real Fiero...With the technology we have now, you should easily be able to build a fully streetable Fiero(With (Relatively) Stock frame/Suspension) that would out-lap the old GTU racer.
[This message has been edited by cvxjet (edited 10-09-2016).]