I just put a new Magnaflow XL turbo muffler on my 76 280z 2+2. I really like the way it sounds (it has a nice low rumble) but it is a little louder than I expected and I'm thinking its going to get the attention of the local authorities, This was an original California car with a CAT. The CAT is totally shot with shattered ceramic and so it is essentially a straight chamber.  Since I live in Michigan (off road country), I'd like to replace it with a small muffler or resonator, The original CAT is 10" long with flanges on both ends. Does anyone know who makes a muffler or resonator that would fit?

stop in at your local mom-n-pop muffler shop, they'll have a catalog with plenty of options. i'd go with a longer resonator (looks like a small diameter, round muffler) at least 18" and have them weld new connections in. and avoid 3-bolt flanges if you can, they can be leaky. very simple work, not expensive and you will be much happier on the freeway. the longer resonator seems to help with drone in the 2-3K rpm range.

I did exactly what Rossi says and love the sound.  Quietened it down and did away with the constant speed drone on the interstate.  $40 at local pop and two sons. 

Thanks guys. That makes the best sense. I do know the owner of a local muffler shop that's done other work for me. My insurance doesn't go in effect until May 1 when I can get it out on the roads and see exactly how loud it is or how much droning there is. But I suspect I will at least need to quiet it down some.