if my meager understanding of charging systems is correct you need a voltage higher than the battery to charge it. meaning your 6v generator won't charge the 8v battery. that might be where adjusting the voltage regulator would come in, i don't fully know.
also don't know if you'd have to step down the ignition system to keep from feeding 8v to a 6v coil and points or any other electrics in the car (windshield wipers and radio for example). 8v through a 6v radio is probably not a good idea.
you could try searching a hot rod forum like The H.A.M.B. im sure people over there have done it. or at least know more than me.