I went through this too in CO.
I bought a used dually to pull the dragster, and it was straight piped. When I got it home, had to find a cat, and install it. The cat alone makes for a good sound, with no muffler. Knocks the "ringing" out of the exhaust note.
By the way, an empty cat can? Most of the inspectors do only a visual, but some that i know of will thump them with a ball pein hammer, and you should hear the fiber wrap around the ceramic core deaden the sound. Just a watchout.
We are an emission station here in Denver at Cummins, and we see a lot of attempts to fool the system. Seriously, the cat does pull 80% of the CO and HC out of the gases, and a bit of the PM, so it does some good. We have tested somoe hot BOMB'ed trucks with a real cat on them, and not too much difference. If you are Real hot on upgrades, register the truck out of the front range of Denver, and you are exempt.