The Fiero is pretty stout! Unless you plan on making the wheelbase quite a bit longer, you should have no problem. Only the roof structure adds anything to the stiffness. From the number of 355 spyder clones using the Fiero, I have not heard of any problems in this regard. A simple 2" x 2" .090 tube skip welded across the roof cut will ensure integrity, but I wouldn't even bother..
I have stretched a couple of Fiero's in the past and that was a no-brainer exercise too.