Good question. The backing plates need to match the drums and shoes I believe. I would imagine the hardware would be the same. Dr. Diff is a good place to start. I bet he can get you all the right parts.
I see the shafts alone are $300, then the rest, new rims, tires, etc. getting too expensive. May need to just keep looking for SBP 15" slots, hard to find but out there somewhere. I don't think I want adapters. I wish M&H still made 14" tires other than 235 6014