Just a thought here: On a vehicle that was built before computers controlled spark, fuel delivery, and practically everything else, most electrical circuits were designed to switch OFF when the key is in the START position, to send the maximum current to the starter. Converting to an electric fuel pump requires the voltage to the fuel pump to be ON even when the key is in the START position. If the voltage to the pump is inadvertently connected to a circuit that is OFF with the key in START position, the vehicle will not start due to no fuel pressure. You probably already know that, but I throw it out here just in case.