Need a Mopar guru: 273 part-throttle ping driving me crazy!

What is your total (all in) timing. It should be no more than 36 degrees. At part throttle the manifold vacuum is quite high, pulling in the vacuum advance and causing the pre ignition (pinging). Are you checking timing with the vacuum advanced disconnected? There are different advance cans with smaller amounts of advance - we need more info here, total timing, amount of mechanical and vacuum advance.

Have you tried the distributor out of the old 318?