I don't have one that early, but I do work in the tire industry and can put in some input. A big factor in what width wheel you'll need is the tire size you plan to run. With a 15 x 6, you really limit the width you can run, 215 or 225 would really be the safe limit. If you're wanting a wider tire, go with a 7" rim minimum. Other things to consider is what suspension/rear axle will be used. That will affect the backspacing and potentially width availability.
