Back in ancient times (that's when I was a kid), my first car was a '51 Olds with a 303 cubic inch V-8 in it and a 6-volt electrical system. Cranking that thing over on winter mornings was a real iffy process, and like Tim said, it had to be done just right, or she no workee. The solution that I found was a 6/12 battery. After putting that battry in the car, it cranked over like a champ. I don't know if they still make those any more, but J.C. Whitney might possibly still have something like that around somewhere. Anyway, what it was, was a 12-volt battery with a relay or shunt or someting mounted on the top of the battery, and what it did was to send 12 volts to the starter motor, then when the engine fired, it would drop back to 6 volts for normal operation of the electrical system. Never had any problem starting the car after that.
I don't know if anyone still makes 6/12 batteries, but it won't hurt to do a little research on it and see if any of the battery manufacturers still offer anything like that. It's also a LOT quicker, cheaper, and simpler than converting the entire electrical system to 12 volts.
