Dodge has been using the same bolt circle for 8-bolt wheels since forever until recently (around '02 or '03). There's a good chance that 16 inch dually wheels made for Ram 3500, early '70s through '90s, will fit. Later wheels (e.g. when the 17" duallies came into play) may be hub-centered and yours probably center on the lugs.
Look for wheels for older Ram 3500, make sure back spacing is the same (or at least correct for the wheel width and tire size) and the wheels mount the same. There should be Dodge 16" dually wheels in wrecking yards.
In this part of the country, I would call Southwest Wheel if I were looking for new wheels and my local tire man can't find something, but usually he comes through for me.
When I swapped out the 8.75 X 16.5 rims and tires on my 1984 Ford Class C E-350. I worked with my local tire dealer, the equivalent metric tire size for me was to switch to a 6” X 16 Ford rim and a 225 / 75 R16 tire. The tire guy ordered two rims (actual Ford OEM new rims) for and two tires. We mounted them up and tired one on the front to make sure the smaller rim had enough clearance on the front brake backing plates and steering wheel well clearance. Then we mounted two up on the rear to check the brake clearance and the space between the dual tires and wheel well. They fit perfect so we went ahead and ordered five more (found a used tire for a spare) and I have been running mine for seven years and it worked out perfect. It certainly gives you a lot more tires to choose from.
Early 80,s up until 1993 dodge dually rims are what you need (off of the dually pick up chassis). These have the same coined stud holes to properly center the rims on your axles . Early 80,s fords had the same style 16" coined rims as well with the only difference being the small alignment hole to properly clock the rear duals is slightly different which is easy to open up .
I did this swap a few years ago and 225/75/16 size tires are pretty much identical to the old 8.75R16.5
p.s. cool looking rig ..... I,ve been thinking of adding that same hood scoop and airdam & wheel flares to my ole dodge to.
car-part.comis a good site to search junkyards nationwide for what you need. That,s where i found my rims at ..... or search craigslist.
Try stocktonwheel.com. Tell them what you need and maybe they can help. I don't think it would be a cheap option, but they're worth a try. Southwest Wheel stopped selling 16" wheels for Dodge 5 years as told to me in am email reply early this year. Good luck on your search. If you come up with a good source let us know.