The coaches in our group were all built with 6*16.5 wheels and mounted 8.75*16.5 tires. A problem, that you should not have but should be aware of, is that not all 16.5 wheels can wear radial tires. The rim may actually crack. There is only one remaining us manufacturer of 8.75 *16.5 (or almost any 16.5) and that is Firestone. While I have had no trouble with the tires I bought in 2008, they were not simple to get and took some doing to get new ones. (Tires age out on the shelf or on the road and that life ends around seven years for most.)
There are 16 rims available for most things that were sold with 16.5, but you have to have the complete wheel specification. I suggest that you look up Southwest Wheel and see it they have a listing for your chassis.
Matt