maybe a little bit of both.
just stating that the perch issue isn't an issue since a B body rear has them in the wrong spot and they'd need to be moved, unless you moved the front spring mount outboard and used offset shackles in the back.....
missed earlier that you said '68-69, so it should be 60.125"....
I did a disc brake upgrade over the winter on my '76 slanty duster, so I'm pondering options (go cheap and get some wheel adapters, Ford Ranger axle as is, or explorer axle narrowed to center the pumpkin, etc). or attempt to find a mopar solution. I had originally picked up a dana 35 out of an XJ, but that was 60.5" wheel mount to wheel mount. based on my measurements, the biggest tire I could fit out back without high offset wheels was a 215 without rolling/cutting the wheelwell lip.