I have operated a fleet of trucks for 56 years. When radial tires became of age about 50 years ago, trade magazines published all kinds of horror stories and for many years a lot of owner operators put bias on their steer even if they had radials on the drive. Radials do give the steering a little different feel than bias tires with manual steering and it takes getting used to if you grew up with bias like I did. A lot of this will go away by giving the radials a little less toe in than bias. 20" tube required tires require a multi piece wheel. The 3 piece wheel was the safest wheel that GMC used for tube required tires. You are very fortunate to have the 3 piece. Many of the 2 tons had the Firestone RH5°. Radial tires do require a tube specially designed for radial tires. There are no modern wheels specifically designed for radial tires because they are a lot of bias 20" bias tires on container trailers. I think calling Toyo will be a waste of time. I don't think any tire manufacturer except for classic tire manufacturers would assume the liability recommending you to mount a commercial truck tire on a 70 year old wheels. Age of the tire plays a more important role with radials than bias than the wheels. Age affects radials a lot more than it does bias. It is important to keep proper air pressure, have radial tubes and have only radials or only bias on a particular axle. I wouldn't worry about the wheel.
