[vettestr]

All engines love timing and some combinations insist on higher initial lead. That being said also beware that preignition-pinging etc.. is in simple terms the explosion of fuel charge before piston has entered its dwell or point that is has begun to change travel direction. The rings in their ring land fight with piston direction and rattle trying to deal with opposing forces. This brakes the ring or hammers out the piston groove they live in.
Pinging can harm engine and if you are comfortable it is being controlled then add timing. Many engines have knock sensors to eliminate this condition via the injection and electronic timing controls. In an engine that is not equipped with this electronic safety net you basicly have to increase until it becomes easy to detect and then remove say 3 to 5 degrees as a safe margin.
I respect those much more well versed than I and admit to taking a bit of liberty in description but beware of harm potential if not looking for the signs and know what they are. Others will explain better or differently but say about 39 total and 15 to 20 initial with delta of 24 to 19 mechanical is what was in most of these beasts on the strip in 66. They had much higher gas octane then with lead in 66 too! Remember to remove from the total if you add to the initial or mechanical because rule of thumb says about 39 is max.