Unfortunately, Catalina used several different engines in the C-320 (I think the original engines were Perkins, then Westerbeke, then Yanmar 3GM30 followed by the Yanmar 3YM30. Each of these uses a different muffler. As a result, the best way to get a new muffler (or to get your old one repaired) is to send the old one back to Catalina for them to fabricate a duplicate.
There was a discussion of this problem on the C-320 International Association discussion forum a few years back. Try their site for more info. (the easiest way into teh C-320 site is through the Catalina Yachts site (
www.catalinayachts.com))
Call the factory in Florida (I don't have the number at hand), and ask for parts. They should be able to help you.
Steve
Alchemist C-30T #4764
Alchemist C-320 #909
Alchemist C-380T #71