Try attempting to start with the high beam headlights turned on. If they go out, or get very dim when the starter switch is engaged, your "new" battery has a bad internal open circuit. If they stay bright or only dim slightly as the switch is turned to the start position, your problem is probably in the starter. By 1969, I believe there was also a safety switch that required the clutch pedal to be pushed in while starting. Does your truck have that? If so that switch could have gone bad.
Jerry
