1995 GMC 2500 Suburban 4x4 7.4l won't p smog. Help!!!!!?

if failing NOX I would think is EGR related as need to reduce combustion temp. to reduce NOX. Could be timing? If EGR system is OK. If all else fails make an appointment to see state smog referee and be prepared to show what you have done as well as total expenses to comply. If local smog guy is clueless as to nearest referee contact local dmv or CARB on line.: www.arb.ca.gov