I rebuilt the carburetor again, and the new kit came with a power valve and gasket of a different design than the last kit I installed. This valve and gasket were clearly designed to work together AND with the design of my carburetor. I tightened the power valve down as tightly as I dared, so if that hasn't taken care of the power valve issue then nothing will. But I believe it has.
It turns out that the power valve was not, in fact, the cause of my hot running problem though. After mounting the rebuilt carb, I was rechecking the distributor advance curve with the car idling in park when it started to miss and quit running. It was then extremely difficult to get it started again. I put my timing light pickup on the coil to distributor ignition wire and saw that it was not firing more often than it was firing. I could not test the coil adequately to conclusively determine that it was bad, but I replaced it under warranty and the problem symptoms seem to have completely disappeared. The bad coil was less than a year old, with less than 6k miles on it.
I'm glad to know what caused the problem, but I am wondering now whether I should expect such short coil life again, and if so, whether it is a poor quality coil or else the location and manner of mounting that's causing the problem. I'm using an Accel Super Stock points coil number 8140, and it is mounted in the stock location--laying on its side behind the carburetor under the air cleaner. Would another coil be more reliable? I'd prefer to keep the stock mounting location. Thanks!
1954 Crestline Victoria 312 4-bbl, 3-speed overdrive