UPDATE: Thank you for all your advice. I had a little time this week to check out everything again.
It would start on the first bump of the starter and run smoothly for anywhere from 40 seconds to 2 minutes. Then sputter, backfire, and quit. Pulled coil wire and had strong bright spark. Electronic ignition recently installed...so no condensor or points problems. Gas always flowed strong and fast from bottom of carb. Exhaust pipe removed to assure no blockage.
After stalling out, I could immediately restart and purr smoothly for 30 to 90 seconds. Then repeat again. Today revealed a potential clue......water in the oil. But no oil in the radiator. Cold this be the AH-HAH! moment?
Today's project was adding one can of K&W block seal. In a day or 2, I'll see how things hopefully improve.