Shit here in Ohio they've setup these nifty self e check systems at the stations. All you do is pull up, throw the plug in the OBDII, and go. As long as you don't have any codes you pass. And it works too. My buddy went through with his Mustang and he's got an o/r and passed. Really kick myself now thinking I spent stupid amounts of money on my Bassani when I could have just bought the o/r version. O well, ya live and ya learn.
On anything pre OBDII your still stuck running through the station and getting the sniffer jammed up your ass end. My Jeep has to go through that. But the Mustang will never have to be taken advantage of by those dirty emissions people...
And honestly since you have a 6, like stated, I'd stick with cats. If you had an 8 it's a different story. My Borla's were soooo damn droney with the cats on that it would give you a headache lol. Pulled the cat's off and it's like a whole new exhaust. I freakin love it. I'll never go back. And being Borla's there not obnoxiously loud. My Jeep's dumped flowmaster is louder, go figure.