First all... cold is your problem. Starting a truck with a cold battery, 6V or 12V it doesn't matter, is tough. Batteries lose their charge in the cold. I'd consider moving the battery from under the cab into the engine bay, or a warmer place in the truck.
Second, your wires are more a problem than the equipment. If your wires are creating a short ("cracked/corroded/just bad"), it doesn't matter which starter, alternator, generator, etc. you have. You won't have power. Make sure you have the proper gauge wires for a 6V system. 6 volt systems require more current, which requires a heavier gauge wire. Get a wiring diagram and check the connections and gauges of each wire.
Those things are rather independant from the 6V/12V issue. But my vote also goes for the 12V & alternator setup. It seems to fit your needs, and you should be able to find everything in Fairbanks.
