Can you see where the spark plug ground strap changes color? It is supposed to help tell if the ignition timing is good?
It sounds like you covered most of the possible issues. I would drain as much of your current fuel from the tank, and put some 110 octane race gas in just to verify the engine will not ping with high octane fuel. Then I would chassis dyno the car using wideband O2 sensors, the see what your air/fuel ratios look like.
Once you get the car in a known good state of tune, then I would reduce the fuel octane an see if the pinging returns.