Only way I could get 275/60/15 under my 71 Barracuda was with modified steel rims, I cut the centres out and fitted them to 8" bands, cant rember exactly but it was very close to 5" back space.
My son and I now do custom rims for cash on the side.

So based upon my clearances, the plan is a 68-70 B Body dimension Dana 60, ESPO +1 6 leaf HD springs, and Bilstein RCD Shocks. Should tighten everything up back there, get the ride height much better, and tuck things under the wheelwells a little better with clearance to spare. I also am aware of the need for new plates and U bolts, and hard lines. Any other considerations?

I noticed a little sag on the passenger side versus the driver's side which tells me those stockers are a little tired.

I verified drum to drum was about 61.5 inches (stock 7 1/4 from what I can tell. Ride height on the passneger side is about 3/4 inch lower than the driver side. No interest in a re-arch because the plan is a 440 and 18 spline setup to a Dana 60 anyway.

I'd be looking for level with you in drivers seat.
I still think you should build a Dana 60.

R.