Vacuum advance units vary in calibration. Some come in at 3-5 in of vacuum. At 1/2 to 3/4 throttle it could be advancing enough to cause pinging.
It would be a simple test to take a test drive with the vacuum to the dist disconnected. It might help figure out what is causing this problem.
I did tune up & drivability work on daily drivers & commercial vans for 50 years. I have found occasionally too much vacuum advance causes pinging.
George
