At this point, I'm most concerned with the occasional 16 and 17 you're seeing. Even 16, where you are not reporting any misfire issues, "might" be on the hairy edge of too lean. Some engines will tolerate mixtures that lean, some will not. Plus, you're measuring 4 cylinders. One cylinder might be leaner than the rest. Before you go any leaner with the MJs, I'd really like you to pull all 4 spark plugs from whichever side of the engine the sensor bung is on, just to be sure everything looks OK.
I agree wholeheartedly w/DaveNC regarding the IABs...go back to the 71s (or smaller), it's quick and easy. You can always pop in larger IFRs if smaller IABs aren't doing it for you.
You're really not far from where you want to be. It appears from your last post that you aren't getting leaner as low-load RPM increases, something I was concerned about.
