PLEASE do not use 8-volt batteries in that system. All that will accomplish is damage the other components like the regulator and possibly the rectifier. The Leece-Neville alternator system is excellent quality, and there should be enough technical information available to get it back in good original working condition. To charge an 8 volt battery, the regulator voltage must be set to 9.5 volts or so, and that's rough on all the electrical accessories. It's a remote possibility your system has a series-parallel switch that routes 12 volts to the starter for cranking by connecting the two 6-volts in series, and then switches the batteries back into parallel, 6 volts for charging.
That's usually used on Diesel engines, but if your rig was sold in cold country, it might have been set up that way for reliable starting.
Jerry
