It was common to change to 8 volts if 6 volts did not seem to start an engine. The 8 volt batteries are probably in parallel which still only gives you 8 volts to the starter. Emergency equipment was often speced redundant. Only one battery is hooked up now. 6 volts will turn a 302 GMC if the starter is good. It probably had some kind of heavy duty charging system. The 8 volt charging would be achieved in the regulator. Are you trying to restore it to original or are you just wanting to make it drivable. If you just want it to run, one 6 volt battery will do. Get a 6 volt generator and regulator or switch it to 12 volts with a 12 volt gen and regulator or simply with a 1 wire 12 volt alternator.