72 dart backspacing question

FWIW, I had SBP 15x8.5 Centerlines on my car (68 cuda) when I got it. They had 4-7/8" B.S. with 255-60 x 15's. The inside of the tires just kissed the leaf springs. A single 5/16" wheel spacer gave it just enough clearance. Those are the numbers that gave zero clearance from tire bulge to leaf spring on my car with an A-body 8-3/4" and stock location springs.

If you stay SBP A-body 8-3/4", a 4-1/2" B.S. should be OK. You will have to determine what rim width will fit though. 7" should be good, 8" ? A 72 Swinger has narrower wheel wells than a 68 cuda.

Later I had C-body axles shortened by Moser to A-body length. With B-body 10 x 2-1/2" drums, the dimension to the drum surface increased exactly 3/8" per side. (I plumb-bobbed to the concrete floor before and after the conversion) I'm using Mopar 17 x 7 steelies and all the Mopar steelies have 4-1/4" B.S. These are actual measurements I'm using.

I wish I could remember the aftermarket wheel mfr that makes a decent assortment of SBP wheels but I'm drawing a blank.

Your results may vary. Chrysler said 1/2" variance side-to-side or car-to-car is acceptable. Best bet is to find someone with a similar car, rear end, etc. and see what worked for them.