The '73-77 A-body r/e housings have a wider spread on their cast in upper control arm perches than the perches cast into housings used in all previous ('64-72) A-body rears, that would mean a different casting of the center housing by an aftermarket mfg like Moser. The typical 12 bolt doesn't have the pinion support of an 8.5 rear, so the extra 3/8" diam of a 12 bolt ring gear isn't the deciding factor in comparing r/e strength. Neither is the extra 2 axle splines, as OEM 12 bolt axles flare up at the beginning of the axle splines. unless the 12 bolt rear has 40+ year old GM HD service pkg axles or quality aftermarket 12 bolt axles, stock c-clip axles are a problem point.
Desire to stay GM rear, one can build up the '73+ 8.5 A-body housing with custom 30 spline bolt-in axles & an Eaton posi unit. I've built several '73 A9 housing A-body rears that way, as well a few with 33 spline spools & Strange axles for track use. What is the Purpose Build of the car, that should be the determining factor. Footbraked 11-12 second street car? Heavy low 11 sec car leaving with on trans brake?
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