Take a look at my post on the wheel well measurements of two Malibus. It looks like the nominal measurement from the drum to the frame is about 5 1/2 inches for a G body with a perfectly centered rear end. So with a wheel with 5 1/4" of back spacing, plus maybe an additional 1/4" for the tire, it adds up to about 5 1/2" of total backspacing . That could just contact the frame in the back.
Scot