John,
Original 6v or a 12v conversion?
If it is 6v, you do not say is you put in new battery cables (positive lead and negative braided-ground to frame).
Most modern cables are 4 gauge but the original 6v cables were 2 gauge and many people use 0 or 00 gauge cables.
Most people have put in the 12v replacement cables over the years. This is the main reason why people who are having starting problems with mostly original trucks think they have to change to 12v electricals in order to have reliable starting
Similar to Stuart's response, I have read that once the starter gets warm, it gets less efficient. This is an added problem with a 6v system and the wrong gauge of battery cables.
Tim
