Fully built cars can still pass emissions if you build them right.
My old 418 ci motor flew through the sniffer with the factory cats and the smog pump hooked up.
The new 422 is a little more rowdy, and I'm having trouble. However, I'm confident that it will get through eventually once I get a good emissions calibration in the AEM.
I've seen cars with ridiculous camshafts and huge heads pass emissions.
Also, in AZ - if your car is 15 years old and insured as a collectible, you don't have to pass emissions. My silver is almost there!
Paul.