RE: EXAUST PIPE INSIDE THE FUSELAGE
Ed can jump in here with details on his method, but what we're both saying is...
If you get air flowing through this thing, you won't have "all that heat inside the fuselage" to worry about. The only time it will be an issue is if you let the plane run for long periods of time on the ground. So, take care of your engine, and minimize ground idle time. (like turbine flyers do)