I grew up around a top-quality independent shop in the 1950's, where we did a lot of charging system work, and 8-volt batteries were pretty common back then. They were the cure to a nonexistent problem. A 6-volt charging/starting system will work just fine, if it's properly maintained. When an 8 volt battery is installed, the voltage regulator must be adjusted to 9.5 volts, which overloads the generator, and burns out lights in a hurry. You'll need to check the charging voltage against a fully-charged battery like I described before, and the max voltage should be 7.5 volts at a charge rate of 10 amps or less. If it's higher, you'll boil the water out of the battery, and possibly damage it from overcharging. Be sure all the ground wires are attached properly, and all the charging system connections are clean and tight. A ground wire from the regulator base to the generator housing is a good idea, ditto on one from the engine block to the body.
There are 3 adjustments on a mechanical-point voltage regulator, closing voltage on the cutout relay, current regulator, and voltage regulator. Most 6-volt generators produce about 35 amps max, and the current regulator should be adjusted to prevent the generator from overloading itself when charging against a low battery. The voltage regulator sets the max charge voltage against a fully-charged battery to prevent overcharging. The cutout relay assures that the generator and the battery are disconnected when the engine stops. If the cutout is adjusted too low, or the points stick, the battery will discharge through the generator when the engine stops, ruining both the generator and the regulator. As a general rule of thumb, adjust the cutout to close at about 6.5 volts, the current regulator to 35 amps, and the voltage regulator to 7.5 volts. It takes a lot of experience to get these adjustments right, so find an oldtime DC system pro if you can, and pay him to do his thing!
Jerry
