When I was looking for a later rear end for my '54 Ranch Wagon the closest thing I found was an 8 inch out of a Maverick. WMS to WMS 56 inches and change. Maybe a Fox body Mustang but it would be 4 lug although you can get 5 lug axles. I seriously doubt you'll find anything with the WMS to WMS dimension you want with the correct spring perch location. Hopefully I'm wrong.
Edit: This site has a bunch of info about rear end widths, etc. Shows a 74-79 Mustang 2 rear end WMS to WMS dimension of 55 1/2 inches. Also lists spring perch measurements. Not sure a Mustang rear end will hold up in your application even if it fits.
https://www.quickperformance.com/Sus...nts_ep_44.html