You sure? I'm not. On pre-1970 (California) Pre-1971 (Federal/Canada) Mopar passenger cars, there's no vented fuel cap; they have fuel tank vents. I think, but am not certain, that the same is true for Mopar trucks.
Nope. Coils can be failing, not just "good" or "dead". If one winding shorts to one adjacent winding, coil output drops a little. If two windings short to two adjacent windings, coil output drops a little more. If three windings...
Maybe. Could also be other factors. A failing coil is possible but I would say but not too likely in this case. Easy diagnostic is to get a hair dryer or incandescent droplight and use it to heat up the coil with the engine cold, before trying to start it. Once the coil's about as warm as it gets from 5 minutes of engine operation, try and start the truck. If no go, replace the coil.
Fuel supply problems within or outside the carburetor are more likely. Sounds to me as if there's trash in the fuel tank and/or in the fuel filter. Engine starts from cold, fuel flow pulls trash against screen on end of fuel pickup pipe in tank and/or against walls of fuel filter element. Eventually enough trash builds up to block flow. Engine stalls and won't restart. Fuel flow stops, trash is no longer forced against screen/element by flow, drifts away from screen/element. Engine will start again. Lather, rinse, repeat. Next time the engine stalls and won't restart, try filling up the carburetor bowl by spraying carb cleaner in through the angled bowl vent tube that protrudes into the carburetor throat. Give it a good long spray; you want the bowl up to its normal fuel level -- might take 30 seconds or more depending on what carburetor is installed. Then go hit the key. If the engine will start, you've got a fuel supply problem.