['ol Pinion head]

ID'ed the rear in post #10. Have several of that style 8.5 housing in my housing racks.
The '71-72 8.5 A-body rear is what I typically build off of, have several dozen, have built up over 300.

'64-65 as well as some extremely early '66 model Pontiac A-body's had the narrow Pontiac 8.2 A-body rear. Aprox 54.5" measured inside backing plate to backing plate.

Next wider A-body rear was hair over an inch wider, aprox 55.5" measured inside of backing plate to backing plate. These were used in many different designs up through the '72 models. With brake drums, 61 5/16" outside of drum, rim surface to rim mtg surface.

'73 up style A-body as well as 77-late '80's B series, are slightly over an inch wider than the previous A-body rears; the '68-72 12 bolt, '66-72 Pontiac 8.2 10 bolts, etc. The '73-77 A-body rear also has the much wider spread on the cast upper perches. The rearend pictured above also has that spread but is NOT an '83-87 G body 8.5 rear.

The G-body rear has smaller diam axle tubes, has its coil spring mounts on the top of the axle tubes, has the thick rectangular tabs welded on the ends with the large hole in the top of them for the brake line to reach the wheel cylinder, the stock G-body housings also accepts their own style of screw on backing plate. The G-body rear is also the narrowest of all above rearends.