As Tiny noted you need the heavier gauge wires if you are going to run 6 volts even for a while. You run the risk of overheating wires and creating a fire if you use a harness designed for 12 volts in a 6 volt system.

I think there is something else going on with your engine if the slowest idle you can get is 800 rpm. These Stovebolt 6’s will normally idle at as low as 450. I typically set mine at between 550 and 600.