The usual width for rear ends on higboys et al is 56".

Measured from outer drum to outer drum which is the accepted way to measure.

Fwiw - I'm running a 55" wide 9" under a 32 highboy.
It runs 10" wide wheels with 285/70R-15 tires.
The four bars are between frame and tire, things are close, but there's never been any rubbing.
Backspace is 2 7/8".

I have a 31 A on 32 rails project roadster that's running a 56" wide 9".
This axle places the 6" wide 15" wheels under the bobbed fenders just right.
Backspace is 3".
Tires are 235/80R-15.