The beauty of the common rail trucks is 400 HP is easy! I would say a TST PMCR or Edge/Attitude, An intake, 4 in muffler or exhaust and you should be at 400+ HP easy. Dont forget about the auto now, a TC and VB would be just fine then a small turbo upgrade, such as a Industrial injection SPS 62 or a 66 if you dont tow and you will be in great shape! Maybe a pusher pump to keep your CP3 happy!