Early 90’s would be the C4 Corvette, which doesn’t look like it had any fiberglass? Something mentioned about injection molded plastics? ::shrug:: In any case, this would be at the most closure panels (things on the surface). You still need good old steel for things like the roof pillars, door intrusion beams, rockers, dash panels (“firewalls”), package trays, and whatnot.
I tried looking this up because I’m interested in exotic body construction methods. It looks like the C4 is a conventional unibody based on the Y-Platform as is the current (yeah, as in today’s) Caddy XLR if Wikipedia is to be trusted.
This means your car doesn’t have a frame in the sense of old chasses you probably think of. Unibody platforms are indeed made of sheet metal. There’s not really any “frame” because all of the integrated pieces serve as a type of frame. There are crossmembers under the floor that act as pieces of the frame, but yet they’re part of the floor. There are rockers and side sills that act as part of the frame, but they’re also part of the body cladding. There’s the dash panel that acts as a firewall, but it’s also built to act as part of a frame. This Google image result kind of highlights what you might call a frame, but really isn’t one.
More recent platforms do include hydroformed tube elements, and it looks like the C5 'Vette has this as part of the front structure.
If you can be positive that none of the sheet metal has been damaged, you should be able to be confident that the “frame” is okay. A quick examination by a body man could you tell if (a) the sheet metal is okay, and (b) that it has or hasn’t been repaired.