[special-K]

Nice truck. I see it's a Custom Camper. The 8.25-16.5 wheel is what the optional 10.00-16.5 tires came on. That's what width is on the front now. It looks like the wagon wheels are 9.75 wide. A lot of guys ran that combo back then,with 12-16.5s on the back. If you mean stock to the truck,check the SPID on the glovebox door. If you mean the stock-type wheel,like on the front,you can find 9.75-16.5 wheels for sale for reasonable and that would look great. The original type of hubcap required inner clips,like old cars ran years before and both those wheels and hubcaps are tough to find. These trucks look great with the wide stock center wheels w/o hubcaps,as most ran them back then. Or,69-75 hubcaps look great,as do the 76-91s. You'd want 9.50s on the front and 12.50s are what you'll find for the rear,unless you go with bias ply from a classic tire company.
If you decide to go to 16",the GM 16" wheels back then were split rims. The later non-split type are 6". What guys like to find are the last SAE Ford wheels from the early-mid '90s that are 7". They look just a bit deeper and closer to what's on front now. Just be sure you are getting the 8 on 6.5s and not the 170MMs.