How far along are you on the restoration? I am doing an 81 Monte Carlo Frame off with an LS1 and 6spd. I am almost done stripping it all down. The Body is off the frame. Going to media blast in the next few weeks hopefully.
To answer your question on rear ends. 1) I am 99% sure that no G-Bodys came with disc brakes in the rear. You would have to either go aftermarket (lots of options available) or you might be able to use the rear disk set-up off a 2nd gen trans am. I think the WS6 cars came with discs in the late 70's, I have no idea if they are easy to find. 2) As far as posi rear ends, G-Bodys had 2 types available 7.5 inch 10 bolts which were available in the Monte SS and 8.5 inch 10 bolt (which are a lot stronger) which were available in the Turbo Regals. I think the Cutlass and Grand Prix all had 7.5, but I am not sure. I think Monte SS came with a 3.73 gear and turbo Regals a 3.40 ish gear.
Depending on your budget you can go aftermarket, a variety of companies offer bolt in Dana 60s, 12-Bolts and 9-inch's.
Last time I saw a G-Body 8.5 in the local paper the guy wanted $800. If you want to change the gear ratio, replace seals and bearings and pay a good shop to set it up, you will end up dropping another $500-$800. You can get one of the aftermarket rears for between $1800 and $2500.
I ended up going with a Strange Dana 60 with Baer Brakes.
Good Luck, if you have any questions as your disassembling PM me.