That's not actually true.
All of the A-body 8 3/4's are the same width (52 5/8" flange to flange). But the BBP 7.25 and 8.25" rear ends are actually narrower than the SBP 8 3/4 rear ends (51.5" flange to flange). And the SBP 7.25" rears are a different width than the BBP 7.25/8.25" rear axles.
Also, if you're looking for a Duster, Demon, or Dart sport you can use a 65-67 or 68-70 B body rear axle if you move the spring perches to the A-body dimension of 43" c-c. The sport body is wider in the rear, so, you can use a wider rear axle. You do have to watch the backspacing on your wheels though, depending on what size you've got on there now.
Here's a link to the sticky on 8 3/4" widths, the list includes some of the 7.25/8.25" dimensions as well.
An accurate 8 3/4" rear axle width list
