EGR cooling?

Its a 82 Fed. Mazda truck 2.0, carb is a NOS stock Nikki 2bbl, 5sp, spark timing is not changeable as it is checked against specs @8 BTDC. Cam timing is dead on as you count links to line cam cog up. I milled the head .060 to clean up some PO's water coolant only on an AL head for pitting corrosion but Mazda spec says .030 is limit and they sell a shim to correct even that. EGR is clean and functional (cleaned valve, no residue but found loads of crap in the feed tube and intake port from the exhaust, complete blockage but cleaned that completely) pintle moves with engine vacuum..and learned that the feather Duster used a larger EGR valve for part of its package than the normal slant to combat NOx levels with its leaner carb jetting, so I assumed more EGR (or cooler) would assist my quest. Carb was untouched since its last test where is passed with flying colors. Changed since last test were a valve job and milling the head. perhaps my cat is done. NOx failed by about 10% over max (ie. 70 max, I had 77) everything else was in line. 250,000 mile truck with new rings (bores straight and smooth) 2 years ago. runs like new (of course) Wife wants me to scrap it, I need my little truck for hauling Moparts! one concern is that the valve timing may be altered with the head milling, as in the cam is advanced slightly because the head is closer to the crank.