You are correct, the chart should have a second entry for the BBP A-body axle length. The one in the chart is for a SBP factory axle. The BBP axles have a different axle flange offset, so that length is different and that is also why the wheel mounting surface measurement is different. You can see the reason illustrated here
View attachment 1715808139
I only added the mounting surface measurements because I use them to figure out wheel backspacing, I never bothered to add the different individual axle length because I buy all of my BBP A-body axles, since they never came that way from the factory.
The original chart should have been more clear, when it says "flange to flange" it means the mounting flanges on the bare housing, and "drum to drum" means wheel mounting surface to wheel mounting surface (wms-wms), like you said the area where the wheel actually contacts the mounting surface of the drum. I probably should have updated it when I copied it from Moparts, but I didn't.
The only differences outside of factory tolerances I'm aware of come from different axles. For example, a factory A-body 8 3/4" (SBP) has a different width than an A-body 7.25" (SBP) for the same year. And a factory BBP A-body 7.25" is different from both the SBP 7.25" and 8 3/4". The same is true for the '71 B-bodies for sure, the 8 3/4" and 8.25" are different widths too. A-body 8.25's are the same width as the BBP A-body 7.25's (or so close I can't tell the difference in measurements).
My own measurements have always shown the housing widths to be very close to the list at the beginning of this thread, I've measured half a dozen different 68-70 B-body axles and found they were all pretty much dead on, less than 1/16" different anyway which can easily be a measuring error if you're just using a tape measure. Same for the couple of E-body, A-body and C-body 8 3/4's I've had in my possession at one point or another. The big differences are all from different style rear axles. Obviously there is a factory tolerance on the housings themselves, but at least from what I've seen that tolerance is much tighter than anything related to the frames or body's on these cars.
