Anyone replace the muffler on their Chrysler 300m? What did you use?

I've been looking into various brands (Walker, Magnaflow, Flowmaster,
etc) and can't really find anything made with stainless steel that
matches the dimensions of the factory OEM muffler.

The 300m has a muffler AND a resonator under the rear of the car. The
tail-pipe comes out the resonator (which looks like a muffler but
apparently isin't).

The original muffler shell measures 18" long, 12" wide and 6" high (oval
profile).

I can find some aftermarket stainless mufflers that are 18" long, but
their oval profile seems to be quite a bit smaller (5" by 8" for
example).

For non-stainless types (aluminized) the Walker 21375 seems to be a good
match - but I'd rather replace it with another stainless steel one if I
can find it.

Any ideas?

Comments?