Problem solved. As y'all thought, the new drums were out of round. Got them turned and everything is working great now. Thanks for the advice, and lesson learned. Always turn new drums!
As to my original question about finding hubs, I couldn't find any at a local yard but found them on car-part.com, from a salvage yard in Missouri. If you happen to need new hubs, be aware that there are two parts out there that look very similar but aren't identical. The hub for a C20 with 13" brakes takes 9/16" studs and looks like this:
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One of the hubs I received from the junkyard takes 1/2" studs and looks like this:
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I'm guessing the second one might be from a 1/2-ton. The only difference is raised metal parts the studs pass through are more square for the C20 hub and more round for the second one, and of course the stud size. Everything is else is identical. Just an FYI.
Oh, and I also have a couple spare hubs now if anyone needs them.