Here's a tip to determine where to cut the hole in your lower cowl for the exhaust stack exit. The problem is a typical "Catch 22". You can't mount the muffler with an uncut cowl in place, and without the muffler in place, you have no idea where to mark the cowl.
Raise the tail on a stand and chock the wheels. Get yourself two pieces of PVC pipe, one that slips over the exhaust stack, and another small enough to slip inside the larger pipe and slide telescope style. Now, mount the muffler and attach the larger PVC pipe to the exhaust stack. Next, slide the smaller pipe downward until it touches the hangar floor, and glue it in place with a glob of hot melt. Now sliding the larger tube down out of the way, remove the muffler, reattached the cowl and slide the PVC pipe back up until it touched the bottom of the cowl. Draw around the pipe where it meets the cowl, and presto! You've got a perfectly marked cowl.