You may need to back the initial back to 10 and reduce the vacuum advance a little. Take a 3/32 allen wrench and pull your vacuum hose off the canister, insert the wrench and see if it locks in. If so turn the wrench counter-clockwise 1/2 turn and test drive. If the pinging stops, go back in 1/4 and see if the pinging returns, if not, stop. It is right. If the first 1/2 turn doesn't do it, go another, and go back 1/4 if it stops. You want to keep as much timing as you can and just stop the ping.

If the wrench doesn't lock, get an adjustable vacuam advance. I have a Standard brand advance for an '84 460 F250. It works great.