Well, you mentioned it only happens after a long drive; generally this is an indication that a component is becoming hot from all of the heat generated by the engien. Not much else changes between startup and after aXXXXXexcept things get hot.
This may possibly be another indication of a failing pickup coil in the distributor; when you accelerate the vacuum advance mechanism is causing the base plate inside the distributor to move.
Take a spare spark plug along with you the next time it is likely that you might have a problem. When it dies, unplug a spark plug wireh and plug in the spare plug, lay it on the metal engine surface and crank the engien over to see if it sparks. If not, then that confirms it is an ignition system problem.
If you suspect you may have a fuel system problem (I suspect you don't but it never hurts to check), carry a can of starting fluid spray with you. When it is not starting, spray a little down the carburetor
throat and see if the engine fires up for a moment. If it does, then it is a fuel system problem; if it does not that is another indication that the ignition system is the likely source of the problem. By spraying some starting fluid into the carburetor throat youa re basically bypassing the entire fuel system, including the carburetor.