My thinking is that if you had any significant vacuum leaks you'd be failing HC as well because the mixture would be off. NOx is simply due to combustion chamber temps being too high. If you're sure the EGR system is functioning properly, try decarboning the combustion chambers. You've probably done it before and know the procedures, but since I'm here and writing already I'll elaborate. The cheapest method is just a spray bottle filled with water - just spray it continuously down the carb throat while the engine is running. A little more expensive but likely more effective method is to pick up a can of GM Top Engine Cleaner (liquid, not the spray) from any GM parts department (Seafoam will work as well but I have had better experience with the GM product) - suck it into the idling engine through a vacuum hose while having someone keeping the engine running. When you're near the end of the bottle allow the engine to stall as you're sucking the last of the cleaner in then let it sit for 30-45 minutes before restarting and killing any nearby mosquitos with the noxious cloud of smoke it will create.