On my Syssa, the muffler is tucked directly behind the engine, which has rear exhaust, and the Pitts configuration fits beautifully inside the GSS cowl with room to spare. The Syssa is an awesome running engine, strong running and dependable, but for a while the folks at Syssa Aircraft had some difficulties keeping up with things, and for a period of time I was withholding my recommendation of them. However, since they've just added an additional service enter to handle the demand, I can recommend them again. The Syssa engines cost a little more, but they come with the Pitts muffler (and the muffler comes set up to accept smoke fittings) and rear exhaust, which makes a nice installation. You can see photos of my Syssa engine install in my GSS in my posts.
Jim