Setting the timing on a 69 318
Read what Crackedback posted (some other good ones too)
Determine if it pings when only partially warmed up
SUSPECT that the timing marks are incorrect and check them with a piston stop
CHECK for severe cam drive wear which may indicate a slipped tooth on the chain
Learn to use a timing light and plot what your advance in the distributor is actually doing. Someone may have modded it or a spring may be broken
FBO (ignition) sells an "advance limiter plate" kit that makes it fairly easy to rework timing advance.
I think AbodyJoe posted an article a long time ago:
Go down to post 28
how much initial timing mild 360 magnum
Also, I believe it's the "FBO guy" believes in "full" vacuum advance (manifold vacuum). I do not and these cars were not designed for it. They were designed for "ported" carb vacuum. When using some distributors with some carburetors, sometimes the ported vacuum comes in too much/ too fast and can cause part throttle ping.