Look at your IAT sensor. Look if it is placed "straight" in the coldair intake or whatever you have in there. The air is supposed to hit the sensor. But if it's in the wrong position, you might get pinging. Last year, on my stock 5.0 the highest timing I could or ran was 11*, on 91 to 94 octane. That sensor is surrounded by some kind of "U". The incoming air was hitting that "U" instead of my actual sensor. Just unscrew it and look at this. Could be it, you never know.