It doesn't work very well to tap off between series connected batteries. It discharges one more than the others and will yield to shorter battery life. It will probably be cheaper to convert everything else to 6V rather than get a 6V alternator, 6V starter. One of the reasons for auto manufacturers to convert to 12V was hotter ignition (even with points). You may end up with a high rpm miss. I can't think of any compelling reason to use 6V (other than stock vehicle, stock engine, stock etc).
