I can't imagine any Pitts muffler for a side mounted engine that wouldn't require some serious cowl cutting on this airplane. The other planes you saw may have had the engine mounted inverted using an "inverted Pitts" muffler. That would probably result in the least cowl cutting. I believe inverted is the standard engine mounting for this airplane. I had one with a .46-AX inverted using the stock muffler and it worked excellent.