Stepside Tire Size?

[javadoc]

Spring is here, and this summer's project is the truck. The first order of business is to install the 3/4 (or 5) drop. However, I know that the rear wheel/tire combo is not going to work once the truck is dropped. I currently have 15x10 chrome smoothies with 295s in the rear. When loaded, they rub. Badly. IIRC, they're 4" backspacing.

I have measured the inner area of the rear wheel wells and it looks like I have right at 12" of width under there, between the bedside and the inner edge of the step fender.

So, my question is, what size tire are people typically able to fit on the rear of a stepside? I'm probably going to have to ditch the 15x10s... but I really don't want to if I don't have to.

Thanks guys, for the wisdom!