As you've already found out, there's no easy/cheap solution.
Years ago I got a set of rims that came off of a late 40's truck that were "2-piece Lock Ring" type 20" rims. I wanted them for a later 1956 1-1/2 ton truck I had at the time. In order to make them work on the later truck I had the center hole opened up to 5.25" (I think). Have you looked at any of the 1940's trucks to see if they have those rims? You might find one in a field and get it cheap just for the rims...
Have you contacted any of the custom wheel shops like Wheels Now?
https://www.wheelsnowinc.com/Mike B
