A Body 12 Bolt in a G Body

I'm sure this has been covered many times here. I'm new here, so I'm not sure if I'm in the right spot for these ?s.
My son has an 80, 355, TH 350, Flowmaster exh., etc. Rather than going thru the expense of getting a GN 8.5, or a 9", I have several 12 bolts, which I thinks he should use. Is it just the upper conversion control arm that we have to get??
What's the backspace on the wheel end being if we use the wider 68-72 A body rear., which I would prefer he use. This is for a street car with occaisional strip time. I have everything except street gears, also if he got more serious about racing I have complete Strange 33 spline setups on hand for these width rears.
Thanks Don

Thanks
On the lower control arms, the stock G body's will work??
Are the A body's LCAs different? as I have a set of aftermarket ones for an A body.
On the wheels, we have several sets of Camaro, MC 7' Rallyes and 8" Vette ones, along with CL Autodrags, and others, 7,8 10s but I don't think any are 5-5 1/2" bs. I guess we better check the wheel stash for bs, just don't want to get into custom wheels, etc. He likes the Rallyes the best.
Don