Thanks for the encouragement, Print Dad
I bought a new PCV hose today. The guy at the parts store said heater hoses will deteriorate, and got me a Weatherhead hose to replace it. He said it should last a lot longer.
So after installing that, I checked my timing again, then readjusted the carb, setting the idle speed and fuel mixture. I managed to get the vacuum up to about 18" (first picture), so I thought, this is good, I solved the problem!
Then I turned off the engine, disconnected my timing light, made sure the distributor was tight, and cleaned stuff up, since I wanted to take it for a drive to check how the power was. But I decided to do one more check before I drove her, so I started her up again with just the vacuum gauge hooked up, and the vacuum was back down to 15"! (second picture).
Still suspicious of the PCV system, I tried pulling the hose and blocked the valve with my thumb (third picture), and behold! My vacuum jumped up to around 17" (fourth picture).
Any ideas? The PCV grommet on the valve cover is pretty new, but that could be the culprit. Other than that, I'm not sure what else to check.
Thanks so much everyone!