A 12 volt alternator simply cannot work with a 6 volt battery. It will either overcharge the battery and/or blow out all the bulb filaments, burn the ignition points, and possibly destroy the gas gauge. Someone would have had to rewire the stator and rotor of the alternator for a 6V output and then fashion some sort of regulator to limit the voltage to a maximum of 7.5 volts against a fully charged 6V battery.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!