Actually, the frame material is whatever GM used at the time, probably a very common steel and about the same as today. During assembly the Fiero unibody, after weldments were completed, was set in a jig and all holes used to to attach body panels were drilled via robot in that one position. All body skin panels were made from urethane, and the car could be driven with no body panels since they were non-structural in anyway. Theoretically and for the year 1984, a superior fit of body panels.
*30 something years ago, there were vids of that car being driven with no body panels.
It was actually a pretty good idea, and if they would have put a decent front susspension in it, insted of the chevette one, it could have been a good little sports car