1975 360 Sunroof Duster

Sounds like a nice find! You should be able to find info on-line about the Space Duster option.
The 8-3/4 definitely did NOT come in that car when new. (Last year was 1972) If it's an a-body rear check it for small bolt pattern. After 45 years it could have a b-body rear or something else in it.
That car originally had an 8-1/4 large bolt pattern, probably a 3.21 gear.