Biggest rear wheels and tires

I am planning on doing a us car tool rear spring relocation to my 72 dart swinger. And I'm also gonna be narrowing an 8 3/4 from a c body. I have a matching set of 15" cragar super tricks for the car when I was planning on a back half, but have since changed directions and just gonna move the springs. The problem I am running into without moving the springs yet just by measuring is I should have about 15 inches of clearance between the frame and wheel well but I have 15 inch wide wheels. What is the next (cheapest) way of fitting them? Or anybody have a pair of 12s to trade?