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

BTW - your issue could be as simple as having the vacuum advance hooked up to the wrong vacuum source.

Manifold vacuum (drawn from below the throttle plates, true manifold vacuum) is quite high at part throttle and will pull in the advance.

Ported vacuum (drawn from the carb, above the throttle plates) will be lower at part throttle as it is closer to atmospheric pressure. How about a picture of the carb and connections?

You can tell by pulling off the vacuum line at idle - at idle vacuum will be present at the manifold port, but much less or none at all at the ported port.