The best options I'm finding for aluminum wheels in the 15x6 range are usually trailer wheels with 0 offset. I did find these wheels from MB that seem to fit the bill pretty well, but I don't know if the 0 offset and 6" width would work. http://www.discounttiredirect.com/direc ... &typ=Truck
I'm currently running these wheels with 185/65/15 tires, and they clear, even under heavy compression tucking the tire in to the fender while turning.

I don't know the size and offset off-hand and need to check them for comparison. But, they look like a common steel wheel from the '70s and I assume they are around 5.5" with close to 0 offset.
My assumption is that if the current wheel is, in fact, a 5.5" with 0 offset, that the 6" wheel with 0 offset and the same tire will still clear as the tire technically won't move. The sidewall will just sit a little different. I just thought you guys had likely seen many different wheel sizes and could easily tell me what would work and what wouldn't.