Isn't that diff 3 inches wider than an A body diff?
The differential is exactly the same. A "diff" is just the 3rd member, even using the slang it's not the whole axle.
The axle housing on a '64 B-body should be 55 5/8", so yeah, the housing would be 3" wider. It gets worst than that though, because the wheel mounting surface measurements are even wider. Factory A-body 8 3/4 with SBP axles is only 57 1/8" wms-wms, the '64 Polara 8 3/4 should be 60 7/8" wms-wms, so that's more like 3 3/4" that the wheels will actually be moving.
Dusters/Demons/Dart Sports are a full 1" wider in the rear bodywork compared to a Dart/Valiant/Scamp, but that Polara axle will be pushing it depending on the wheel choices. I run a 68-70 B-body 8 3/4 rear axle in my '74 Duster, it's 60-1/8" wms-wms and I'm running 7" of backspace on 18x10" wide wheels with a 1/2" spring offset to make that work. If 17/18" wheels are the goal then there will be options that have enough backspacing for wider tires in the back. If 15's are the plan, you won't find anything off the shelf with enough backspace to run a tire much wider than a 225.
I ran 225/60/15's on 15x7's with 4.25" backspace on my Duster for a bit with the 68-70 B rear, they barely cleared the quarters. The Polara axle if it matches the dimensions in the sticky will be an extra 3/8" per side wider, so factory wheels would be a no-go. You'd need a significant positive offset to run any kind of wider tires in the back, and there just isn't much in the way of big positive offsets for 15" wheels without going custom.
If it were me, while I was moving the axle perches to match the A-body dimensions (or a 1/2" offset, might as well, gotta move the perches anyway) I'd be inclined to shorten that housing, weld on new housing ends and get rid of the tapered axles and brake drums and make it a more manageable width all at the same time.
