As stated I would not recommend running the cone inside the pipe, the exhaust will not generate the venturi effect required to pull cool air into the pipe and keep temperatures reasonable, I would use at least one inch from end of cone to start of pipe (not including the intake ring radius).

Still, you may want to wait and get some responses from other members. or you can get some ideas from here: http://www.rcuniverse.com/forum/m_8341245/tm.htm