There's a difference. The corvette was crash tested using the fiberglass body and therefore engineers could design a structure that would meet impact requirements.
Fox Mustangs were designed with steel fenders, and with their uni-body construction (fender is part of structural integritity), changing to fiberglass could have disasterous effects if ever in a frontal collision.
Ever see a Mustang with a fiberglass (not urethane) front bumper in a collision? It shatters basically to nothing as there is NO play at all.