It's been a long time since I had mechanics class, but the best I remember from the dark ages was the the ballast resistor dropped the voltage to around 9 volts, maybe even 10 volts in some applications, not 6.
A vehicle that has been converted to 12 volt will need a 12 volt coil and the resistor. Running a 6 volt coil may work for a short time, but from what my memory tells me (and it might be wrong) it won't last long.
If things have changed I'll be glad to learn, but I know folks that have tried using a 6 volt coil on a 12 volt system even with the resistor and it didn't last long.
