I agree with Tim. Even if you spend the extra cash on a 6V alternator, you won't solve the problem, and the only reason converting to 12 Volts makes a difference is that there's twice the voltage to push the current through loose, dirty connections. Spend a little time with a low-reading voltmeter finding the voltage drop points in the circuit instead of throwing money at the problem.
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!