That's not a G body rear axle. Sorry, but I've seen too many. Finding an 8.5" differential is uncommon (round cover indicates 8.5"). But that alone doesn't rule it out. What definitely says "No way" is the shocks mounted in front of the rear axle and sway bar attached inside the rear control arms.
Here's a G body axle:
I agree with joedoh that we're looking at a B-body (Caprice/Impala) chassis.