Timing woes

I posted this on the Squarebird site also. Have a 1960 with 430. having a ping on take off under load as climbing a hill. I set the timing at 6 and it pings hard. Set it at 3 and it pings a little less. I set the timing, drive it and check it again and the timing mark has moved(advanced). I set it with a vacuum gauge, with the vacuum at 20 and the timing mark at minus 5 degrees. No pinging and the car runs strong. The 4bbl kicks in and the car takes off. Am I safe running the timing set at that mark and does anyone know who rebuilds a damper. What are the signs of retarded timing and when will I know when it is too retarded.
Thanks