The other piece of your original question, that I neglected to answer, asked whether modern tubeless wheels were available to allow you to get rid of the multi-part rims. The answer is yes, but finding them would be difficult. Tubeless truck wheels were introduced to the market in 1956 having your bolt pattern in both 19.5" and 22.5" sizes. All of these wheels, however, have a 5.25" center hole. I'll defer to Grigg and the others to tell us whether your originals were hub piloted or stud piloted since my references don't tell me that. But the fact of your center hole being smaller, if hub piloted, would mean you'd have to have hub adapters made to use these newer wheels (assuming you could find a set).
The other issue is rim width. The earliest of these tubeless wheels were quite narrow at 5.25". This would make them visually similar to your stock lock ring wheels, but would create tire issues. The 22.5" tubeless is technically the correct replacement size for a 20" tube type. The narrowest 22.5" tire available today is a 9R22.5 which is too wide for that rim. It is spec'd for a 6.75" wide rim. I've heard of guys using this tire on the narrow rims, but the manufacturers didn't recommend it. And you'd probably have to use spacers between the rear duals to avoid tire rub.
The use of 19.5s would be the other option, but again I'll defer to the others to say whether you'd have drum clearance issues with these. Technically, a 19.5" tubeless wheel is the equivalent of a 17" tube type. Below is a side-by-side diagram of a tube type and tubeless to show the differences in rim design, and to explain why the tubeless and tube types have the same outside tire diameters while having numerically different rim sizes. The diagram compares 22.5" and 20", but the same concept also applies to 19.5" and 17". Stu
http://i286.photobucket.com/albums/ll89/truckdog62563/wheels/profilecomparison.jpg 
