1972 swinger tire sizes, ansen sprints

Ok folks, just a quick question. I have 3 15x8 ansen sprints with 4.5 inch bs. Should I get another 15x8 or should I get two 15x7 with 4 inch bs? What size tires are people running with a similar setup? I have a ford 8.8 cut to stock size and hd springs in stock position. Also have pst torsion bars up front.was thinking about 275-60/15 in the back. would these fit stock location springs on a swinger? any help would be greatly appreciated.