Does it ping only under load, or also ping when holding steady at those rpms in a low gear? Are you using high octane gasoline? Have you tried octane booster, enriching the fuel mixture, reading spark plugs? Did you change jets when you put the exhaust kit on it? Possibly going up a couple jet sizes would take care of it.
Hmm, pinging is too lean a mixture or the timing is too high, or you need to run premium fuel. Did you re-jet the carbs after putting the exhaust on, if not, that is a good place to start