An 8 volt battery is a solution to a nonexistent problem. The voltage regulator will have to be set to 9.5 volts to charge it, and a "6 volt alternator" would have to have a custom-made solid state regulator to achieve that voltage. Your "free" battery will end up costing you about a thousand dollars to build a one-off charging system to keep it charged, and headlights and other 6 volt items will have a drastically shortened lifespan. You'll also need a custom-specification ballast resistor to keep from burning out the points on a regular basis. Good luck!
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!