The '58s were 12 volt factory. I have a suspicion that perhaps either there's a loose connection at the ignition switch (green wires on the switch) that go to the coil, or perhaps the switch itself may not be making good contact internally. On the starting circuit, there as a heavy purple wire that come off the starter that bypasses the resistor, giving full 12 volts to the coil. This is only engaged during starting when the pedal is depressed. That's why it's trying to start and seems to want to run with the pedal depressed.
