I don't think a 4.5" bs will work unless you use a spacer. I have 4" BS and there's only about 1/2" of gap max. between frame and tire, BUT I'm using street tires..
If you are running 15x8s with a big slick the tire is much more bulged and will rub the frame. Less bulge on a 10in wheel but the fender lip comes into play.
Kevin, I was the guy that just bought you kit last week by the way.
I have 15x8 with 4" back space and 275-60 DR. I'd say 15x9's will work, but 4.5 BS or 4' + spacer is a must. I don't have aluminum drums. I wanted my wheels tucked in close as possible. I now have 3/8" to 1/2" between frame and tire and they don't stick out. If they made my wheel in a 9", that's what I would have bought and played with spacers.
If you are running 15x8s with a big slick the tire is much more bulged and will rub the frame. Less bulge on a 10in wheel but the fender lip comes into play.
Kevin, I was the guy that just bought you kit last week by the way.