No need to go beyond the tail.
Is ok with about 0.5 ~ 1 inch before the end, and cover that area with BVM heat shield.
This way it produce a diference in pressure that induce an cooling air flow inside the fuselage.
If you are using a double walled pipe from Tam ( the best), you can eather put the pipe flush with the rear of the fuse, but I consider is better not to do it.
Contact [email protected] , he will be happy to assist you.
Good Luck,
Miguel