By putting it on the dyno, I'm assuming that they test for NOx in MA, so I would suggest that you not run lean. Running lean means running hotter. Running hotter increases NOx emissions which may lead to failure. Just make sure your car is tuned up and timing is stock. Make sure that the EGR and air pump is working. Go in assuming you'll pass. I wouldn't invest a lot at this point in trying to pass. If you fail, figure out where you failed and fix it, but I wouldn't sweat it at this point.
Your plan to run it lean is a good example of fixing one thing and breaking another in anticipation of something that may not even be a problem. You may reduce CO and HC emissions but increase NOx emissions in the process.
Disregard this message if they condemn your car and impound it upon failure in MA. My state would just tell you what the likely problem is, have you fix it, and then test again. Fortunately we don't do testing anymore.
------------------
351W 89 Mustang GT Convertible