You must have a 1 ton rear to fit those 18" wheels. I tried to get 19.5's to fit over my 48 rear drums to no avail. But if that were the case, they would be 8 holers, wonder what kind of set up you got? Any pictures?
If you had all 20"ers, the 8.25 x 20 tires are still plentiful and easily had,
Scott