i just did the tire swap last month went from 8.75 -16.5 to 235 -85 -16s, my rig fell just barely into the hub piloted age.
the 235/16s lowered my tire revs per mile by 50 revs per mile not a lot but should help with fuel mileage a bit.and also they are rated at 3042 lbs each much better then the 16.5s
16.5s are still around both firestone and goodyear sell them,in my neck of the woods about $200.00 a pop but they have to order them in.
a 225-75-16 is slightly smaller then the 8.75/16.5s if you have the room the taller tire will help in the gear ratio.
look on line there are web sites with pics of all different rearends that will help you identify yours but i would hazard a guess that 4.11 is the highest gear offered as the big rear ends are geared toward work trucks you might be able to find something out of a later diesel dually with gears into the 3s but I'm just guessing on that.
the easiest way would be a gear vendors overdrive unit but they are very pricey new i don't know about out of a rv salvage? i'm thinking about doing some digging into this when i get time it's out of my price range right now.
keep after it though would way cool if you can get it your swap to work.
best wishes frank,cricket and harley the dog