Dave,
I don't have to worry about the emissions here. I've replaced the two stock mufflers with Smitty's 18" glasspacks (they've been around at least since the fifties) and the exhaust sounds way better than stock, although you can tell by the sound that the exhausts are coupled through the third CAT. There's sort of a restrictive muffled groan at higher RPM's. I'm going to hack off the third CAT and install two more Smitty's in place of it to provide enough back pressure and give it a true dual exhaust system without the expense of the X pipes. It should also improve the sound more dramatically as well as eliminate any droning sounds. These two sets of 18" mufflers will total 36" of muffler per side. I have two 40" Smitty's on my 64 Ford F100 with a 312 "Y" block and it sounds awesome, but not loud enough to knock your socks off. Also, the 40 inchers would never fit under my Mark. I know that there are after-market companies out there that sell exhaust components for our Mark's, but after listening to all of the sound files (companies and members) I've found on both of the Mark VIII sites, I not a happy camper. They just don't sound like V8's to me. If I'm not happy after this MOD, I'll try something else.