I'd measure both axles, add the numbers you get and add another 1.75-2". That should get you the rough mount to mount surfaces. If it's in the 57.5 range it's most likely an A body rear.
Funny you would say that. When I was measuring the rear, for the heck of it I did measure the axles. If you hook the measuring tape on the axle flange of the axle while it is laying on the bench, and extend the tape at a angle to the very end of the axle I got 27 7/8". So that would put it at 57 1/2 to 57 3/4 inches.
That with the perches @43" and they look like they were born there, I'm sure it is a A-body 8 1/4. It was the 11 x 2 brakes that threw me for a loop.
