Your best bet for reliable success is to use either a Jet Model Products or BVM fixed length pipe. Either of these pipes provide optimal performance with your setup with no fiddling or guesswork. Neither are cheap but will be far better and less trouble than any other setup.

I had a fan powered Starfire once upon a time that had your engine and fan in it. I believe it was setup with the BVM pipe.

http://www.jetmodelproducts.com/exhaust.htm

http://bvmjets.com/Pages/not%20used/exhaust.htm