Are you sure it's a ping and not a mechanical noise from inside the combustion chamber? I hope that's not the problem, but your description points in that direction. Before doing anything else, you should check everything, especially the cam timing. Then pull the plugs and turn the engine over by hand to see if you feel any resistance. Then turn the engine over with the starter. If you hear the noise, the problem is inside the combustion chamber.
At this point I'm sure it's not mechanical. It doesn't start til the motor approaches operating temp then progresses from a slight tic to a heart stopping klanging. Shut off the motor, wait 45-60 seconds, start again and it's quiet for maybe a minute then the progression starts again. Run it with the plug wire disconnected and you stay quiet.
Just a shot in the dark: Could it be that the exhaust valve is sticking while open when tolerances get tighter due to hot exhaust gases? Was this exhaust valve and/or its guide replaced during the rebuild?
I suggest you use a bore scope and check the piston for any marks of impact. If it were an ignition timing issue, you could see it with a scope.