Emissions Failure - NOx

I just picked up a 1994 Firebird with a 3.4l. The car had a DTC 76 which is EGR when I bought it. I replaced the EGR with a junkyard unit, the code went away so I proceeded to take it for an emissions check. It failed the first time in HC 2.50 with a .80 limit and NOx with 3.50 with a 2.50 limit, CO passed with 9.00, 15.00 limit. Long story short, cat was gutted, replaced it with a Walker unit, HC passed with .73, same .80 limit and CO registered 0.00. NOx failed even worse the second attempt, 4.50 with the 2.50 limit. Where should I go from here? The plugs looked a good tan and the car seems to run good. Other than that, no other parts were checked or changed. Thanks for any input.

Nox is created from the combustion chamber being too hot,it is either running lean or the used EGR valve is malfunctioning.

When was the last time your o2s were replaced?

I'm sure the o2s are original @ 136k. I ended up putting a new EGR valve on and that did the trick, dropped the reading down to 2.5ish. The HC and NOx are still on the high side for a stock 3.4l imo, but this will have to do. Thanks for the input.