If the plan is an 8.25" rear I would stay with the 4.5" backspace. With the large tires you may need a small spacer, but I think you'll be close enough that the spacer would be less than a 1/4" anyway. That car has highly modified quarter openings, like super stock style. And there's a ton of rubber outside the quarter. If you're planning on cutting the quarters out it will work, otherwise you'll need to go substantially narrower than that.
Going with less backspace doesn't really help you, at least not once you get past about 3/8" of clearance to the springs.
