You won't like the results of running it on that short pipe.

Cut the airframe apart and extend the tunnel to fit the pipe in with the 11" header and tune it the proper way.

Don't take this the wrong way....... if you don't know that a tuned pipe must be "tuned" then you probably want to stay away from them or start doing some reading in this forum. A tuned pipe is NOT "bolt-on" power. It has to be tuned. That means checking RPM and then selecting the right prop and RPM range. Then adjusting and tuning the header/pipe system to perform best. Using a header that is too short will lead to a burned up engine if you run it too long. Thats no joke. You are taking a very severe risk running that engine with a pipe thats too short. It effects the pressure waves and sound waves inside the pipe. It won't be tuned right.

Sincerely dude.... do some reading or get the longer header. Or you could just forget the tuned pipe and just slap a pitts muffler on it. Just don't run it like that with the 8" pipe. You'll probably damage the engine.