The 78 and 79 "A Bodys" are the first "G Body" cars. GM changed what the platform was called, not how they built it or what parts they used.
Any how, I beleive you're stuck to looking at later 80s GM cars, specifically the GN, T Type, Hurst/Olds and 442. The monte SS had a 7 5/8s" rear ring. and the others were 7.5. I'm not sure if they even had 8.5s in the A Body cars.
G body.org says-
" First, ALL GM 1981 to 1988 G-Bodies used a 7.5" ring gear rear axle EXCEPT: Monte Carlo SS - 7-5/8" 3.73, Olds 442 - 8.5" 3.73, T-Type and Grand National - 8.5" 3.42 or 3.73. "
Differentials - Code Listing
Hopefully someone else will chime in and verify that i'm giving you the right advice.