Ok, my guess is the vacuum advance is adding too much advance. Try driving it with the vacuum advance disconnected and vacuum line plugged. Crane makes an adjustable vacuum advance for the HEI.
Vacume advance ADDS timing under high vacuum. Some add too much, or too soon, or both. The Crane adjustable advance has a adjustment to alter this.
It could be as your vacuum comes up it adds advance and thus the pinging. I had to add an adjustable vacuum advance for this very reason. Simple fix.
