Unless there's something WRONG with your HEI, you've already got what you need. Limit the centrifugal advance with a different center plate; or by welding/brazing the slots in the centrifugal advance mechanism. (on the top of the distributor shaft)
If it'll throw enough spark to jump the gap of an HEI spark tester; and the pickup and ignition coil are the right ones for a Pontiac, (clear plastic connector on the pickup coil, and the red and WHITE wires on the ignition coil) your "hard starting" is probably not a fault of the distributor.
HOW is it "hard starting"? Won't crank? Cranks slow? Cranks fast but won't run? Stalls after starting? Cranks and backfires? (double check your pickup coil and ignition coil!!!) Something else?
http://www.chevelles.com/techref/ftecref5.html