72 slant 6 plymouth duster will not stay running

All of these guys are right if it's the ballast resister the motor will run whilst holding the key in the start position. Then cut out when you let the key go. That's because to start you get an extra boost with full voltage to the coil.
Another thing to check is the strainer in the tank blocked up this happens if the tank had fuel in it and slowly dries out. That can leave a dry film over the fine mesh. It could even be a split diaphram in the fuel pump. That you need to fix as it will result in bearing corrosion.
Or a simple blockage in the carby.
If you have put on a fuel filter or pushed a rubber hose onto a metal fuel line someware check that as a piece of rubber can get sliced by the metal tube and the tounge of rubber can block the line. I hope this helps.