The axle width chart doesn't tell the whole story.
The drum flange to flange width is the important one
Early A small bolt pattern rears are narrow but, small bolt is worthless to us.
Big bolt pattern adds an 1 " to each side, making a big bolt A body rear almost the same as a B rear, which most of us use for narrow rear ends.
People over the years have done things you would never believe.
What is the reason for the question or what are you trying to achieve ?
Just a narrowed rear end ?
Lot more to consider than just housing width.
I've got this listed on the A Body sight. [FOR SALE] - 8-1/4 rear Flange to Flange is just over 56". I also have an early (65-67) B body 8.75 I have not yet listed for sale, but will be soon.
(Don't shoot me Mods - just trying to help with A body rear dimensions)