I'm having the same issues here.
I have a float with 10.00R15 bias ply tires, 15" split rims, a 4" wide spacer ring, all held together with 6 wedge clamps on 6 studs. I'm pretty sure the setup was completely standard 40 years ago.
I blew a tire a while back, and since the tire was unavailable, the dealer upgraded me to 11R17.5 radials, on a special sumitomo rim that fits on the original hub.
Problem: the 4" spacer ring allows the tires to touch. Dealers tell me there is no wider ring available. 4" is the widest.
Any ideas?
This seems to be a topic where pictures are worth a lot. Wish I had one handy.
Ken