I had symptoms just like this with my '51 Chevy 3100. I purchased the truck in Alabama, where it is nice and humid and moist, just like Louisiana. My problem was corroded battery cable and ground strap connections. It would start just fine, but as current went through the bad connections the resistance created heat, which generates more resistance, until the darn thing would not start. Let it cool, and no problem, again. Remember, assuming your truck is still 6 volts, the current draw is twice that of a 12 volt system and requires clean connections.
Disconnect and clean (wire brush, etc.) each battery cable connection, including your ground strap from engine to frame. This should take less than an hour, and even if it is not the cause, you won't have to worry about this for awhile. I fixed my problem by doing this and have never had a problem since in 10 years.
Let us know how you do,
Kent
