Rear diff question

IIRC
At 4 inches narrower, and with the springs relocated, the housing will be too narrow to put the perches in the correct location. One U-bolt on each side would be on the bearing housing, and that won't fly. You would have to suck the springs towards the centerline, to make it work; and I cannot imagine what that might do to the bushing life, or the handling.

To fill the wheelwells up, you will want about 8.5 inch rims with about a 4 inch backspace.I'm guessing a little there.