I hope you got a california specific cat because they can fail you on visual for having a non stock cat without a CARB number.
Seriously, find another smog shop (preferably pass or don't pay type) have the guy really warm up the engine first. I took in my 18 year old mercedes for smog with 166k on the clock. In the pre test the smog tech said the car was going to fail because HC was too high. The owner of the shop jumps in the car and revs the engine to 2500rpm for 1 minute then he tells the guy test it again. The car passed with extremely low HC numbers, in the single digits.
Pretest
HC max 120, measured 135
Actual test after rev to 2500rpm for 1 minute
HC measured 8