Thanks for the feedback. I passed the test in the meantime. This is what I did:

- disconnecting A/C vacuum hose to the cockpit since there seems to be a vacuum leak there.
- found an other vacuum leak in the charcoal canister line.
- Disconnected smog pump so that no air is feed into exhaust system (not sure if the system leaked air).
- cleaned ignition rotor/distributor
- changed "R16" resistor to a lower value to retard ignition timing
- changed oil
- put 2 bottles of ISO heat into almost empty fuel tank
- drove the car for 30 min on the interstate in 3rd gear prior to inspection

NOx is right on the limit at high speed, and about 10% below limit on low speed.