8 volt battery in 6 volt system was discussed here awhile ago because of the charging system..6 Volts just don't spin the engine fast enough unless everything else is right..I have changed the hole system to 12 volts. With less volts it take more amps to do anything. So your 6 volt wire size is good .Unless the wires are fraid. Your 6 volt starter will work with 12 volts. I've been using 12 v on my 8N for 5 or 6 years now. no problems and my dump truck (6 volt) has been on 12 volts for 15 years.just change the bulbs .