[motto]

Lionel, if the wheels on your truck are the stock, standard Chev 20" type of that era with a lock ring that is full circle, (not split like a cir clip) and they are in good shape then they are quite safe to use.
As I posted on another forum recently, GM and Studebaker between them built around 800,000 6x6 trucks during WW2 (These used the same lock ring arrangement as the Chev 20" wheels we are discussing). This gives a total of 8,000,000 wheels. Add another 800,000 spares and those produced for other trucks such as the Yankee Joe and we must be up around 10,000,000. Yes that's 10 million 20" wheels.
I have yet to hear of one failing.That doesn't mean that it hasn't happened in the intervening 70 years but if so it would be an exceedingly rare event.
As regards the question of what license is needed to drive this class of truck it would quite likely vary from state to state. In Victoria you can drive a vehicle up to 4.5 ton Gross Vehicle Mass on a car license. Strangely enough you can also tow a trailer up to 9 ton GVM subject to the capacity of the tow vehicle.
I have a friend that was pulled over by an RTA official whilst driving his Studebaker 6x6 and asked for his license. The RTA bloke said that the truck could not be driven on a car licence as it exceeded 4.5 ton gross and the friend tried to tell him it was only a 2-1/2 ton truck and pointed out that it said so on the dash plate. The limb of the law didn't agree with that explanation.

David