Hi,
On my 1975 F-350 I was able to buy some 16" rims on Craigslist for them. The bolt pattern was the same.
Your rear axle is probably built by Dana, and is the same bolt pattern as my 75 Ford was. Chances are that you can find a F-250 - F-350 in a junkyard with 16" rims that will work fine. Look for years 1985 to 1999. It was a few years later when Ford went Metric, and changed it's bolt pattern to something else.
I would be looking at 6.5" wide rims and if you have single rear wheels then 245 mm wide tires, such as the 245/75R16E that is used on the E-350 vans. In fact vans from 1985 to 1995 should use the same bolt pattern. But buy one first, then try it on your van before buying more.
By taking a careful measurement from one bolt to one on the other side, you can compare the bolt pattern. If you have 8 holes, then the F-250 - F-350 bolt pattern should be the same. However if it is 6 bolts, then you are pretty much on your own.
Fred.