I had a similar problem on my 58 burb with a 250. Turned out the two nuts holding the carb to the intake worked lose. I had rebuilt the carb and reinstalled it without lock washers. Ran fine until the engine heated up and things loosened up a bit. Then the truck would die in the most inconvenient place. While your checking everything else, put a wrench on the carb nuts and make sure they are good and tight.
