Don't mess with the catalytic converters. Flowmaster makes some good dual exhaust kits for those cars. The best bet is to buy a full catback and install it. They can sound pretty decent and you're only looking at $500 - $600 for the kit, $100 install or so.

That's a fantastic find.  Those old Mercury's are solid running cars as stated above.

Since Scotty mentioned them some time ago, I've been half-heartedly looking for a Marquis or a pre '08 Crown Vic. 

I found a 2006 Merc offered at an estate sale with only 48k miles.   Too bad they wanted $12,500 for it....unreal.  And it sold for that price.

A few years back, Borla offered a cat-back that sounded good (to my ear at least).  I didn't look to see if they were still making the kit.  It was probably pretty expensive though.

https://www.youtube.com/watch?v=iBZby_UahZ0

Hey Scotty. I've owned over 70 cars in my life and just bought one of my best. It's a 2003 Grand Marquis with only 63000 miles on it. Love the car, runs great but I want to upgrade to duel exhaust. Not sure about what to do with those four cat converters .  Do I need them all ? Not asking about the law, just how it might effect the oxygen sensors or anything else. 

Congrats on finding a car you love. Those were the last decent years of GM.

@mod_man can help you with the exhaust stuff. He knows all about that.

Cadillac converters are merely to burn unburn hydrocarbons they serve no other purpose. Of course it's breaking the law if you take them off

Your question at 8:24

https://www.youtube.com/watch?v=Zju4hzJYeHU