The 12V option is a good choice. Just disconnect the supply wire to the ammeter at the starter to protect the rest of the electrical system, and use a jumper wire to the coil + terminal instead of going through the ignition switch. As long as you don't crank the engine for more than 5-10 seconds at a time, the 6V starter will handle the overload OK. If you put a ballast resistor in series with the jumper to the coil, the points will thank you!
Jerry
