Passing Emissions Tests
High NOx is usually due to a stuck, plugged, or disconnected EGR valve. A stuck/inoperative EGR valve commonly causes knocking because the added exhaust flow the valve puts into the cylinder will cool off combustion, and reduce the chance of knocking. The level of knocking must be too high for the knock sensor to reduce.
Typically, a failed EGR valve shows up as surging and/or knocking at part throttle, but the knocking can occur any time.
Before you go messing with new and different injectors, replace that EGR valve (or check all of the hoses/connections). With a high NOx reading, an EGR problem is pretty certain. Check all of the vacuum lines that lead to the valve (it might be working fine, but the vacuum to operate it might never get there. A test to see if it works is to idle the car, then suddenly open the throttle to 2000-2500 RPM. You should see the EGR valve stem open up as quickly as the throttle did. If it doesn't move or is sluggish, there's the trouble.
As for being tough to remove, the valves often don't rust in place, because the heat passing through the valve dries everything out. It might be tough--just go easy so you don't break any bolts. I've never done an EEK EGR valve, but the factory installed one on my '78 Volare came off very easily last year, after 19 years and 140,000 miles. I was very surprised.