Do you have enough accessories to need the extra amperage of an alternator? About the only vehicles to use the alternators originally were police cars, ambulances, and fire trucks, etc. There's absolutely nothing wrong with running a 6V generator as long as it's set up with a properly-adjusted regulator and clean, tight connections.
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!