You might look it up by his name, Mike Ostrom it came from a 1972 Mustang complete from drum to drum located in Phoenix,AZ $300
In the listing he states it is 54" backing plate to backing plate, 61" from the outside of the drums. Measurements are in the "Sticky File" https://www.jalopyjournal.com/forum/threads/faq-sticky.897329/ See Post 9. Using later model Ford wheels that have more offset will gain you some room. You can compare that to the popular 8.8" specs here. https://www.therangerstation.com/tech/ford-8-8-inch-rear-axle-history-and-specs/#gsc.tab=0
When I replaced the stock rear axle in my 55, the stock axle measured out at about 58" with the brake drums on at the wheel mounting surface. I used a Granada axle (was told it was a 1976) that measured the same 58" width. The spring perches were within a 1/4" and dropped onto my springs without modification. Needed new u bolts with longer thread as the old u bolts wouldn't tighten completely as the nuts bottomed on the thread. Easy bolt in. At 61" wide I think my 15X6 wheels would be rubbing the outer wheel well lip so too wide for me.
The Jalopy Journal
