Problem with all of that is that it's used with an unknown history. One or more of the previous owners could have worked it like a rented mule, or worse. More air and fuel will make anything make more horsepower, that's the easy part. The hard part is keeping it together. That's the reason the 6.5 has been revised and outright redesigned up to the P400 spec engines, and those were only rated to 250hp/550lbft. If you can show me how to make a reliable 400hp 6.5 without sinking Dmax or Cummins swap money in one, I'm all ears.