I was wondering if anyone has seen or herd of a better sounding muffler for a 66 6cyl mustang than the one that came stock. I was thinking of putting a new muffler on to make it sound better. Just wondering if anyone has any idea.

 

Just about any quality muffler will give you better sound - Flowmaster, Magnaflow, Borla, etc. I don't know if they make a "direct" replacement, but I'm sure one of their inlet/outlet styles will fit.

Are you planning to add a header to it? Maybe a split header that will go directly to dual exhaust? That, with a new free-flowing muffler will give you better sound and a little more hp. In my opinion, you want to keep a 6 cylinder fairly quiet though - it will never give the same rumble as a V8 no matter what exhaust you put on it.