You should try what truckernix suggested first. If then you do need to replace the cables then the forum by Randy Rundle under tech tips says the original 6 volt cables had 4ga wires from the battery. You can also do as he suggested and what I would do is use 1ga which is a bigger wire. To better understand it the cables act like pipes in a water system. The bigger the pipes are the more water will flow. The smaller they are the more it will restrict water flow. The water in the case of your truck would be your amps from the battery. I would suggest to replace both the negative and the positive. This will create even current(amps) flow from the battery.
Chevy 1956 3200
"So she said, "either I go or the truck goes!"....Hmm, I wonder what she's doing these days?"
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