Corvette radios were unique in size and shape up to 1976. While they looked somewhat similar from '68 up to other GM cars, the distance on the controls was wider and the radio was much short (not as deep). In '77, they redesigned the dash so that a conventional sized radio could fit, to reduce costs associated with a specially made radio. This allowed the AM/FM, AM/FM stereo, and the AM/FM Stereo with 8 track to be available, which had been available in other GM cars for several years.
So the answer, none.
The only options you have are:
- Corvette radio '68-'76
- custom radio design, such as those available from many Corvette vendors like Mid America, Eckler's, etc.
- Cutting the dash to fit a modern radio