1991 Dodge Truck starter

This truck is a D150 w/318, recently it was parked for the nite and next day the starter only clicks. So far I have replaced the voltage regulator, the starter relay, the starter {I also manually checked it when the problem persisted} and the battery. I have checked or had checked the alt. And the body & block grounds still only 'click' from the starter.
What else would prevent the starter from engaging?