MT Drag Radials

What width rims are you all running with MT drag radial 255s? I was thinking about 15x8 rims for my 67 Dart that I'm getting a B body 8.75 and doing a spring offset kit for. Surely someone has done the exact same thing I'm thinking about. I'm trying to pick out rims I like, but without knowing the backspacing I'm pretty limited. I think I'm going to need quite a bit, like 5.5+, but they don't seem to make many that big that I like.