acme,
The vacuum advance unit should have zero play (truck not running) and if you take the cap off and twist the rotor, it should move with moderate effort, and spring positively back to a rest position.
The timing mark should only move in the advance direction in reference to the pointer, but should not go more retarded than its at rest position.
Sort of hard to understand I know..using a clock as a reference, at idle, the mark is as 12 o'clock. As you increase RPM, it will move 1 or 2 or 3 o'clock. As RPM decreases it will move back to 12 o'clock. BUT it should never move to 11 o'clock or 10 o'clock.
Find the problem first..make it run with the ignition it has, THEN experiment with the HEI or Pertronix or whatever. Keep at it.
Stuart
