Steve,

I would start by laving the drums off, and putting the 2 flat head screws in. Then make sure the hub still turns freely. If so, then the interference is the drums. Then put the drums on and slowly tighten the flat head screws while rotating the drum, to see when it binds and hopefully rub marks will tell you what is hitting.

The other thought is that 70 year old trucks have had a lot of repairs through the years. Originally It would have had Huck brakes, but did someone change them over at some point? And make sure you put the correct replacement parts in, a lot of chain auto parts stores do not "know" the parts, they just give you what the computer says fits, which isn't always correct.

Jon