Jim, I did a lot of research to find a
suitable muffler for my 15-cwt truck. In the end I fitted a generic Bosal muffler, albeit a different one which can be fitted to the Carrier.
When searching for the Bosal 190-115 Citroën muffler I found the dimensions to be listed as 37.6 x 24.5 x 21.3 cm. If I were you I would take those dimensions to a well-sorted auto store / exhaust shop and find a suitable product in the USA.