As you guys probably know, the real problem is to have enough spacing from the leaf springs.
My car, with totally stock steel wheels and 185/60-13 DOT race tires (Toyo RA1s, Yoko A032s, or Kumho V700s)will lightly rub if I do not use an 1/8" wheel spacer. I'll admit that my tires do "bulge" a little bit...but not as bad as it sounds.
My stock wheels have 3.75" backspacing. You really can't go any more than that with any wheels unless you buy those "curvey" leaf springs (Jeff knows what I'm talking about). I'm not sure you can even buy those anymore.
On the other end, if you run 2" backspacing, you may fry your rear wheels bearings, even if you have the double-bearing jobs. And the extra offset of a 2" backspace might bend the stock rear axle housing (I bent one last year).
If I were to take a WAG, I'd say a backspace range of no less than 3" and no more than 3.75".
Jeff, Hap, Tim and some of the other guys may have more exact experience with the wider rims so I'll be interested to hear what they say.