So I could safely run the 255's on the 15X7 rims with 4.25 back spacing ? With 3:91 gears I want to try to get as much rubber down without the half inch shackle off set.
I would say cautiously that you should be able to, but, it really depends on your car. Toolmanmike does it with that combination, but with the clearances you'll be at it pretty much comes down to body tolerances. I've seen a bunch of cars that have made that combination work, and I've seen a few that had rubbing. The left side of your car being narrower could be a little troublesome depending on where that 1/4" of lost space is.
You should be at about the minimum safe clearance, but, even an 1/8" in the wrong direction at the minimum clearance means it rubs.
If you wanted a better idea before you pull the trigger, measure the distances from the wheel mounting surface (the face of the rear drums) to the quarter lips and the springs. A carpenter's square and a tape measure work well in combination to do that so you're pulling measurements that are all in line. Then you can run the math with the section widths and backspacing to get a better idea of how much clearance you'll have and where. You can cheat the springs down to about 3/8" clearance, but the quarter lip needs a 1/2" minimum.
