Shot in the dark here, but one of my old van's ('87 4WD auto trans) also didn't have a cat-converter. Needing to pass DEQ prompted me to do the same as you, but problems arose with the oxygen sensor when I installed the converter, resulting in it running poorly. Of course, I never figured out what was wrong but instead registered it in a different County to avoid the emissions test! But regardless, if memory serves, the O2 sensor is after the cat-converter so just throwing the Cat in is very much going to change what the sensor sees... maybe? best of luck!