Well kitcarguy just sumed it up.The thing with a Fiero based replicar is its mainly for the looks, you can't really go that fast unless your willing to spend alot of money reengineering the chassis.
Go with the tube if you got the money, everything will litarly bolt on. No stretching, or widening the suspention.
However there really isn't a Tube chassis avalable for the Countach based kits unless you buy a Prova kit(which is no longer around) other than that I would suggest using an 88 Fiero GT or forumla, its got Lotus tuned suspenstion and brakes. Its more powerfull than the typical Fiero and you may or may not need a V-8 swap, bolt on a turbo and there you go.
Although it sounds easy but keep in mind the Countach is still a very difficult car to replicate as well as any lamborghini of any year. Only a few of them ever see the light of day where they are complete while fewer are replicated accrurately, where talking wheels, interior, color and even sound! proper badging, proper headlamps, proper wing extc... they want a spsific model they stick with it.
Ultimately they don't cross bred an SV with a 30th.
A Countach on the other hand is black and white, there's a few of them on the market still to this day and most of them are the 25th anv model and hardly anyone else offers them. So building one accrately wouldn't be as difficult as it would with a Diablo. I wouldn't recomend a non stretched model, they look very funny and cartoony, for that you might as well keep the fiero the way it is, your better off putting the time and the money in for better results.
Good luck.
I just can't imagine driving a 20 year old F-car