Holley main jet numbers do not represent diameter. They represent flow rate. Changing the jets 2 sizes up or down will make a noticible change in AFR. I'd have to look up that list number but 66 PMJs are relatively small to begin with. Take it up cruising around 65 mph in top and you'll know if its too lean or not. Don't do this on a crowded road or some place you can't slow down and pull over.