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I've learned a lot of details about the Chrysler ignition in our B23Fs, so I may be able to help out here. Your mention of high NOx and pinging popped out like keywords to me.
You should check all the vacuum lines, sealing areas (around the intake manifold and injectors), and the big 'accordion hose', but pay specific attention to one vacuum line running from the intake manifold, down under the front of the block, into a little black electronic module on the passenger-side fender, just in front of your coolant tank. This black module is the ignition control unit, henceforth referred to as the ICU.
This little wonder was made by Chrysler. It basically senses two things, RPM and vacuum. Under normal running conditions, the unit tries to advance timing as far as it can (up to 52*) until it detects pinging. This happens almost as soon as the throttle is pressed, and tends to create a leaner running condition, and possibly high NOx readings. Volvo had trouble with this, and actually performed a recall to place 'vacuum delay valves' in this vacuum line to slow the computer's response to changing conditions. Unfortunately, these valves cause driveability problems (like excessive ping) as they age and get turned around & stuff by inattentive mechanics.
If you see ANY green & black or black & white valves in this vacuum line, I recommend you remove them and run one straight vacuum line from the manifold to the ICU (you can just pull off the ends of the old one if you want, and run a new one where you can actually reach it!) I'll bet your pinging problem will be history. Of course, we'll assume your baseline ignition timing is properly set to 12* of advance already (it probably is.)
Now, to pass the test. Your car will drive much better with the direct vacuum line attached, but you may have still end up with a high NOx reading. To avoid this, you can remove the line you just installed and plug both of the ports on the manifold and ICU. This puts the ICU in a 'fail-safe' mode, where it advances the ignition timing based only on RPM, much like distributors did back in the 60's and 70's. It is a very gentle ignition curve, so you shouldn't have any lean-running conditions. In fact, if you prefer, you can leave it like this all the time and drive around just fine! You may notice a little jerkiness when lifting off the throttle suddenly, and the power will be slightly reduced, but it is certainly driveable. Assuming you don't have any bad vacuum leaks anywhere else, and your O2 sensor is working, and your catalytic converter is healthy, you should pass with flying colors this way.
Let me know how it works out.
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Speed Racer, '83 240 R, '74 164 E, '93 940 OL1 (Manchester, CT)
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