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I would go with 7" of backspacing. I'm running a 10" wide wheel with 5" of backspacing. The section width is slightly over 12" and the tire fits nicely under the truck, with about 1/4" to spare from the sidewall to the inner fender lip. If you run a 12" wide wheel and 7" of backspacing, your tire should fit just like this, depending on the actual section width. If you find that it's tight, you could always roll the inner fender lip to gain up to 3/4" more clearance. You most definitely will need to tub as you described to get that wide of a wheel/tire combo in there.Here's my thread that contains more than a few pictures. http://forum.73-87chevytrucks.com/smforum/index.php?topic=7505.0THanks for the reply. We will see how things go. You got a sweet stepside. Like the wheel and tire combo..