I've got the 6v starter and am using 12, it works fine. I've heard if you crank it for extended periods you'll burn it up, so I never use it for more than 5 seconds. Since my engine turns right over, no big deal. It's when you are tuning it or first starting it after a rebuild you have to keep it in mind.
As far as the ingition resistor in the coil circuit, or ballast resistor, or voltage drop-they're all the same thing- That is used when you keep the 6v coil, points and condensor. You can upgrade those three things to 12v if you want to, or you can get a '12v in 6 v out' coil, like the ones VW used in the beetle.
