Peculiar stalling problem

My guess is that you have a heat soak problem on the suction side of the pump, a vapor lock of sorts. When the system is cool it works fine, and as long as you have cooling fuel running through the suction line, things work properly. When you stop the engine and let things sit for a while, heat builds up in the system and you develop a situation that causes you to lose suction on your fuel pump. It cold be an actual vapor lock or possibly a vacuum leak that only happens when the system is warm. Your car runs far enough to empty the bowls in the carbs and then it quits. You then have to sit long enough to allow the system to cool enough to alleviate the problem.
 
You can test your fuel pump suction by connecting it to a vacuum gauge and cranking the engine. There is a specific vacuum/pressure gauge just for testing fuel pumps. I would do it right at the pump when the engine is cold to get a baseline number. When your malady occurs, plug the vacuum gauge into the fuel line right where it leaves the sending unit and do a similar test. If it checks out okay, I would examine the fuel pickup/sending unit very carefully.
 
I was able to replicate the problem this weekend in my garage (with tools and shade). I removed the fuel pump, disassembled it and reassembled it and then reinstalled it. Now all of a sudden the problems seems to have disappeared. It has not stalled since. I'm not sure exactly what I did, but it did seem to have solved my stalling problem.
 
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