52 5/8 is suppose to be outside to outside. Now with that said I have built a many of A-body 8 3/4 and have found them a 52 7/16 to 52 11/16. I don't think quality control was that great back when they were being built.
Yep QC sucked. They were all over the place, usually within 1/8" either side of 52 5/8"
52 5/8" is the general width to go by for flange to flange.
The info Joe posted you can't go wrong with. If you swap to BBP axles using that bearing standoff, it makes the drum to drum about 3/4 wider in total than the std 57 1/8".
You don't have to fight with the center hole to find center to center on the perches. Have the tape grab the outside edge of one perch and measure to the inside edge of the other, that will get you a good measurement!