how much backspacing do i need?

Depends on the wheel width. I can tell you I was running a 15X10 weld aluma star wheel with a M/T 28/12.5/15 on a 72 Demon it had 5" backspacing and I had moved the leaf springs in with the 3/4" relocation kit on each side. They may have fit with out that mod as I had about 3/4" clearance on the springs to tire. But I also had disc brakes that pushes the tire out another 1/4 to 1/2" from the brake hats

If I had to guess a back spacing for your application I would say 4-1/4" on a completely stock car. If the tires hit the springs or inner fender you could always use a small spacer to move the wheel out a bit.