The 5.72" bs mentioned earlier is what the Mustang bullit (and several other) wheels that some members have used. Unless you run spacers in the back or a B-body rear end, that much backspace won't work in the rear. They would be just about right up front however. Ideally, with stock rear, you would run a 5.70" to 5.75" bs in front and 4.75" bs in the back. This based on a 17" wheel.
Keep in mind that the 8.25" rear end is narrower than the 8.75" and would require even less backspace.
You need to get your backspace and offset measurements dead-nuts if you want to run a 255 or larger, assuming no rear wheel well modifications have been done.