catalyst questions - 3.2 swap into '77S
 

I swapped a 3.2 into my '77S. It is stock, with ~5k miles since major overhaul. Runs great. No cat now though, just bypass. I need to visit CA BAR to get approved. As a precaution I went to a "test only" center for a check. It fails a little on CO and alot on NOx. Our host lists a DEC aftermarket cat for 1/3 the oem price. Cats from other places are half the DEC unit.

So my questions are, what is an aftermarket cat? Do they get an old one and test it somehow? Do they install new catalyst? Any btdt out there? Is any cat capable of taking taking NOx from 1400 ppm down to 500 or so?