12bolt swap
If you're spending the money to have the rear shortened etc. you can probably afford a Moser, Currie drop in rear.
Anyway, the lower control arm mounts on the stock '64-'72 A body rear will work on the 'G' body, It's the uppers that are the problem. There is a G body conversion control arm set up on sale at craigslist.org, central Jersey. You can make the stock uppers work easier if you get a set of hop-stop bars from Jegs or others. This moves the upper control arm mount point up towards the floor of the car. Good for traction and easier to move around the arms(after you add new holes about an inch forward of the old ones) If you have adjustable uppers you may have enough room to adjust. Lotsa luck. Steve