distributor shaft play

Be careful how you check the end play of the shaft; you can't check this from the top by pulling on the rotor or top shaft, because you are pulling on the mechanical advance plate and upper shaft, not on the main shaft. This needs to be checked from the bottom with the distributor out. With all new bushings and such, the main shaft will have an end play of the main shaft on the lower end of the distributor in the .007-.010" range.

Pinging is more likely what you are hearing; it is very high pitched like tapping a very tiny hammer lightly on a hard surface and is erratic. Detonation usually is much harsher and with a deeper tone.

Is the carb from the '76 also? If so, it might be from a Dart Lite or Feather Duster, and the carb was calibrated very lean; my '76 Dart Lite pinged normally under certain moderate throttle positions at lower RPM's just like you are describing.

The '76 distributor for these particular models had a different advance curve too. Is the vacuum advance connected to the ported vacuum on the carb? And are there 1 or 2 vacuum ports on the distributor's vacuum advance dashpot?

But I am more suspecting that that the carb is just leaning out, since the timing is not effecting it. I would troubleshoot that by putting a vacuum gauge on the manifold runner to cylinder 6, and look at the vaccum at which this pinging starts as you add more throttle and when it ends as you add even more throttle. The vacuum readings where it starts and stops should say what carb circuit is being activated when the pinging starts and you can investigate that. Feel free to report what you find for vacuum levels.