JDL,
I had a similar problem on my 65 Panel Truck, it turned out to be the battery cable. There was nothing wrong with it visibly, but we used a set of charger cables from the battery to the starter and it turned right over. Replaced the cable and no problems since, sttripped back the insulation on the old cable and about 6 inches backf from the tterminal end the cable was shot. Just a thought.
Mike
