RE: Top Flite Giant Scale Spitfire ARF
I am a big fan of using silicone header couplers to extend the exhaust tubes. They also will soften the exhaust note considerably. I get them from Mac's Products for the most part. Any other silicone tubing should work just as well. I usually put a very small sheet metal screw on the tube and then use a cable tie on the other side to clamp.
FWIW.