You can go two ways on this problem. You can buy the flexible exhaust extensions that slip on over the muffler stacks and clamp down with a zip tie, or you can purchase some metal tubing that will fit into the opening of the exhaust stacks, cut it to the length you want, and use either solder or JB WELD to attach them. Mind if I ask which PItts muffler you are using? I am running the OS 1.60FX on my Goldberg Pitts 12 with a Brison Pitts muffler, and the stacks stick out of the cowling a good inch. Another bit of info for you... a bunch of people say that the OS 1.60FX wont run well with a Pitts muffler unless you crimp one of the stacks closed, because the muffler wont develop enough back pressure. I would suggest running your engine first and see how it does. I lost count of how many guys told me mine would never run right with both stacks of my Brison muffler open, but it has run like a charm since the first day I installed it, with no back pressure problems.