I would absolutely Love it if I could replace the 20's on my 50 4100 with a 19.5 steel wheel/radial tubeless tire. If a 19.5 cleared the rear drum, I would think with modern machining it wouldn't be that difficult to make a wheel with the right center bore/bolt pattern.
About 2-3 weeks ago I did replace mine with a brand new set of 8.25-20. The. Old ones were cracked and "out of round". The ride now is much better and most importantly, safer. Firestone were available, but I hoped for a foreign brand to save a pile of money and still have the project going. When I got the truck, it was mounted on 7.50-20, Wich, in my area are impossible to find with out giving out an arm and a leg. My rims are 6.00 X 20, three pieces in good condition. If you have a bit of knowledge in mechanical work and tires, don't be affraid of tackling them, just respect a few safety rules, and make sure all the parts are clean and in good condition. You just need a duck bill sledge, couple of pry bar and soapy water. To inflate them, I first set them with about 15 psi, then mounted them on the truck, then fully inflate with a remote valve and gauge. In the back, if something goes wrong, the rings are facing each other, and the front, a few loops of loosely wrap chain can do the job. Just make sure everything set and seat good, if not sure, or not right, release all the air, and reinflate. Go for a ride around the yard if possible and reinspect. I went with the original style to keep the original look. 19.5 are ok, but are several inches smaller and, in my book don't look right. It is an old truck, with old suspension type, and what we can barely called a seat for today's standard, so unless you upgrade EVERYTHING, even with radial tires, it will still ride like a stone boat. Just my 2 cent.