New guy,
Unfortunately they are definitely the Firestone RH5's. My trucks use the same 6 lug Budd pattern as yours. I have found that this lug pattern was used on many Ford, Dodge, IHC, GMC 2 ton trucks and buses from the 1940's to 70's. Widen your search and you should be able to find some safer split rims. Find your nearest old timey junk yard and look at the old school buses that the owner uses for dry storage...those buses are often great donors.
Others have mentioned the 22 inch tubeless rims, but I have had more luck hunting unicorns.
