When tuning the pipe, you usually shorten the header 1/4" at a time until the engine speed stays the same or drops slightly, then pull the pipe out of the coupler a little to recover the RPM. I've run pipes with up to 1/2" "gap" with no problems. Use nice large wire ties pulled really tightly to secure the coupler and you'll have no problems. Use a pipe mount at the middle or rear of the pipe to secure it to the fuselage.

Dr.1