I may have a solution to your problem extending the exhaust tubes on your Pitts style muffler. I recently completed a Christian Eagle with a O.S. 1.20 4C engine. The engine is side mounted which allows the muffler to stay within the cowl. It is pointed down at an angle and toward the firewall but stops a couple of inches away from the firewall. I purchased some silicone tubing 3/8" ID X 5/8" OD and zip tied 8" to the end of the muffler. The tubing bends slightly as it exits the bottom of the cowl. Your installation might be neater with a Pitts style muffler. The tubing will probably come straight out. If the ID works on your muffler I can send you some. Just let me know how much you need. Otherwise, you can look for a different size at www.usplastic.com.