In rear with small bolt axles redrilled to large bolt, I’ve run:
235/60/15 with 15x7 4.25” backspacing
225/60/15 with 15x7 4.25” backspacing
245/50/15 with 15x8 4.50” backspacing
* if you’re running aftermarket large bolt rear axles, 15x7 I’d run 4.25 backspace rims.
In the front with large bolt 73-76 disc conversion I’ve run:
225/60/15 with 15x7 4.25” backspacing -close
245/50/15 with 15x8 4.50” backspacing - very very close
* I would never run 3.75” backspace in front. 4.25” backing or 4.5” in the front.