welp... lets get a little more into this then
do you have your full readout numbers??
high NOx can be caused by high combustion temps... specifically temps exceeding 2400 degrees...
that is usually caused by: high compression, lean mix, or overly advanced timing
its an LS engine, so compression is not the issue
i trust it is a stock map on the PR4, so its not a map problem... the thing to look for here would be vacuum leaks
you retarded the timing all the way and still failed, so id say its safe to say its not the timing either
so im going to say you are not likely failing in the combustion chamber
so where is it coming from?
high NOx is generally a sign of a bad Cat or EGR
the 94 LS engine doesnt have an EGR... so thats not the problem
you said the Cat is new... did you get it up to temp??? you should take the car on at least a 20-30 minute beat ride before testing... seriously drive the hell out of it... you want to get the cat good and HOT so that it actually works