Take your 400 , drop a 520 jug on it with the Thumper Racing piston , put the 520 SX cam in it, no jetting needed. Now you have a short stroke 450 with the close ratio 6 speed that every one loves ! I have 4 years of Racing ( Enduro/Harescramble) and thousands of hard trail miles with no real problems eccept a couple of small problem oil leaks ! I have not even put new rings in it! More miles of smiles than any other bike I have owned!