I am currently going through the same problem. I tried 15x7 with 4.5bs, but the tires were only 8" tred. It cleared on the passenger side, but rubbed on the drivers side (barely.) I am looking to get 15x8 with 4.5"bs, but I'm sure spacers are needed with any sized tire on those wheels. I am learning that Weld Wheels don't make the 15.8 with 4"bs, but Jegs Prolites and some Greg Welds do. I haven't found anyone that has said yay or nay on the 4.5"bs with that size tire.
I am running an 8.5" rear end with aftermarket axles. The measurements on my stock rear end and the 8.5" are exactly the same. I'm hoping someone will post what the biggest wheel tire combo you can fit with the backspacing is. I don't want tire hanging to the outside, right now, the wheel is more important. 4" or 4.5" with spacers.
Please help us out!!!!!!!!!!!!