Hi,  I am curious as to how soft or hard this car is driven on a daily basis.  "Ping" is when the fuel/air mixture preignites and could be due to a couple of things...
1. Excessive carbon buildup on the tops of the pistons or combustion chamber that actually raises the compression ratio.
2. Hot spots on tops of pistons or combustion chambers that stay hot enough to fire the mixture just like a spk. plug.(doubt it in your case)
3.  Tried any different gas stations lately?
4.  Ignition timing too high, is your mechanical advance in that HEI distributor working?
5.  Problem with carburetor that leans fuel mixture during that particular speed/throttle opening..

The first thing I would check is the mechanical advance of the distributor.
Next, go to your local GM dealer and purchase a can of "GM TOP ENGINE CLEAN".  Pour half a can through slowly with engine running in between 2000 and 3500rpm and let the engine bog and shut off. Let sit 30 minutes and start up(may have to hold pedal to the floor) and pour the other half into the carburetor with engine running at the same speed as before.  The more smoke out of the tailpipe the better...

Good Luck, Chad

I agree with Chad, with one clarification:
Your distributor does NOT have mechanical advance.  It is fully computer controlled.  I suspect the pinging is caused by the above mentioned carbon buildup deposits which can be dislodged by the cleaner Chad described, followed by the classic “take it on the highway and blow the carbon out!”

At highway speeds (about 45 mph) fully open the throttle through passing gear, up to the legal speed.  Do this several times.  You should see the typical brown cloud of carbon and fuel from the exhaust.  When the exhaust is clear, it is gone.

If this doesn’t do the job, then its EGR valve time.  Some of these engines used backpressure to actuate the EGR valve, and if the passages get plugged, the valve will not operate.  To check if the crossover is open, run the engine and with your hands, lift the diaphragm off its seat.  The engine should die, or severely stumble.  If this doesn’t happen, the intake manifold or heads are clogged with carbon and need to be opened mechanically.

The EGR control may also be inoperative.  Have there been any check engine lights?

Regards;

Franz, MASE

Good advice guys. Let me add that this engine also has
a knock sensor probably located on the back of the pass.
side cly. head. To check it Bring the engine to about
1800 rpm & watch the timing mark while tapping on the
intake manifold nearest the sensor as possible with a medium sized hammer - The timing mark should jump. If
not replace the sensor. While at it you can check the
contition of your timing chain by removing the dist. cap
and turning the crank back and forth while observing
the ign. rotor. With 138,000mi. you probably have nearly
20 deg. of play. This could be your trouble also.
Replacing the timing set now will prevent you from getting
stuck somewhere. It's time. Hope this helps