I would go with a S463, S464, or S466. Your stock fuel is going to limit you to a out 600hp anyways. Bluecollar made 700hp on a S463!!!
http://www.duramaxdiesels.com/forum/showthread.php?t=58467
I can't really lower it any more, because the compressor housing will hit the injector rails and line. These turbo's are much bigger than an S400 turbo!