As I recall, it started with fiberglass (determined it was too laborious to make) then to injected molded plastic (cheaper to make but didn't survive and warped badly with the help of the sun) then back to fiberglass. This is off the top of my head without looking it up.
Supplies from vendors for various parts changed too, steering wheels, hoods, pods, traction bar covers etc. Hand built cars, gotta love'em.
Cheers,
~Earl J