E body to A body rear end swap

Ben is correct, that you use a 3" wider rear end you can counter that with 1.5 inches more backspacing on each side.
With all the custom Mustang wheels out there it isn't too hard to do.
We had a narrowed dana in a 67 belevedere, springs moved inside the frame. Still had to de flate the slicks, w/ 16" slicks , we had to take the shocks loose and let the rear dangle , w/ no air in the tires to cram them in. There are more than one way to skin the proverbial cat. Even took the bolts lose in the spun alum. wheels and turned them around, because we couldn`t afford the correct rims for that set up. Car went 10.40 on the best runs on a terrible, barely prepped track, "back in the day". On a few runs it held the front wheels about 6-8" in the air , till I pulled second gear. Never ever forget that feeling !!