Wow, that's an interesting one. I'm tempted to think it's a (very) bad wheel bearing...maybe...?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Wheel bearings are nearly new, with no visible damage or wear, and no noise when I drove it a week ago.

would an incorrect installation of the primary vs. secondary shoe have such a spectacular effect? but you've been driving it on the same breaks before this started, right?

Im with Dan on this. Even though the wheel bearings were replaced 1000 miles ago IT REALLY sounds like a wheel bearing. There are alot of Cheap bearings out now but even a good brand can have a problem.
My02.
Frank

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Scrapple: Because a mind is a terrible thing to waste.
73 Duster - Race Car
66 Dart Wagon - DD
178" FED
82 D150
All Slant powered

Should be fairly easy to check...slack off the wheel bearing preload adjustment such that the drum stays in place but isn't locked down by the nut , and see if the behaviour changes from "wheel won't turn" to "wheel keeps turning but bearing obviously seizes".

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

After checking everything again, I have to agree that the bearing is the only logical answer. I ordered a new set from Rock Auto and will get the drum turned before I install them. At least I'll know that the wheel and drum are stable and turning true.

A side note...I was going to order a new drum, and discovered that they had drums for both 2" and 2-1/2" brakes for a Valiant with the slant engine. I thought they were all the same.

Mine are 2-1/2", but that drum isn't stocked, and the note said it could be up to 31 days to get it, so I'lll use the old drum for now.

The fronts are 9″× 2½″. The rears are 9″× 2″. If you're keeping the 9″drums, it's best to use the much meatier 70-'72 finned drums, which give you a bigger margin of heat rejection before the brakes start fading. See this post.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Update for anyone interested.

I had the drum turned, and it was within FSM tolerance, according to the machine shop.

Good old Rock Auto came through again with a quick delivery and the right part numbers. Unfortunately, both bearings were made in China. The seal was USA made, but I sure wish it had been the other way around.

I cleaned up the old bearings and gave them a close inspection. There was a very slight discoloration that was apparently caused by overheating. I also found that the outer bearing cage had a tiny deformed area on one side. It wasn't enough to bind aroller, but it might have contributed to the problem. FWIW, the bearings I removed were made in Romania.

Compared to some bad front wheel bearings I've seen, the ones I removed looked pretty good, and they never made any noise. However, something caused them to get hot enough to discolor the metal.

After adjusting the brake, I took it for a short test drive, and the problem did not reappear, but we'll see what happens.

Turning just one drum is a bad idea. If the front drums are more then 0.010" difference in diameter the car can pull to the larger side under heavy braking.


TopHat

The drum was only turned enough to determine if there was an out-of-round condition, but FWIW, finding a forty-year-old car with drums within .010" of each other has about the same odds as winning the lottery.