I'm building a SIG 1/5 scale J3 Cub with a Magnum 52 4 stroke motor mounted on it's side. I need to add about a 3/4 inch extension to the muffler to clear the cowl. I got a piece of Brass tubing from my LHS that fits snug on the tip of the muffler, my question is how is the best way to secure it/ I thought maybe a # 1 or #2 sheet metal screw or drill a hole through the muffler tip and the brass tube and run a piece of thin stainless wire through and twist the ends together. Any tips or thoughts would be great.
RC Stout

The drill hole and a metal retention of some type would probably be best

If the muffler is fully enclosed in the cowl an extension tube of the same size as the exhaust outlet will probably work. I would attach it with a small sheet metal screw. Your cowl should be fairly easy to remove so that you can check to see that the screw remains tight after each flight.
One of my main complaints about 4 stroke engines is that I have never been able to find an extension that fits between the muffler and the engine for cases such as you mention. I suppose there are such mufflers, altough I've never found one that will allow the exhaust to be completely cowled.