Smog test Failed for 95 240sx....?

First of all, that car is pre-Obd2 so you will never see a check engine light for a bad cat. If it was Obd2 they wouldn't be using the tailpipe test, they'd only be scanning the car. Secondly, a new catalytic converter is not 2-3 hundred just for the part as we install them all day long parts and labor for 240.00. That high of a Nox reading indicates a problem with the cat or egr system. I would go to a shop that has a digital thermometer, warm the car up, and take the temps at the front and back of the cat, the rear should be at least 75 degrees hotter than the front when the car is in loop mode (warmed up). You will need a vacuum pump to determine if the egr is working properly.