As others have said, be sure that you have proper gauge cables. I had exactly the same symptoms with my 51 half ton, original 6 volts. Wouldn't start after a good run. It turned out all I had to do was clean the cables and engine ground strap connections. Haven't had a problem since I did that several years ago and the six volt system starts like a champ every time. Even if they don't look too bad clean them anyway, before you contemplate changing anything.
