I myself have a 04 with the H.O. cummins and I love it. There's nothing on the engine itself identifying it as an H.O. but if I remember right if the 8th number on the vin is a "c" then that meant it was a H.O. Also if I'm correct the high output cummins only came with a 6-speed. I searched high and low to find everything I wanted on mine. Also I do believe they started putting emissions on midway through 04 but I could be wrong. I get about 22-24 MPG with mine going down the freeway but I have a pretty heavy foot and love to listen to that turbo whistle! 2007 was a split year for the engines themselves which by that I mean you could buy one with both the 5.9 and 6.7. But after that only the 6.7 was available. My dad has a 6.7 and I'm pretty astonished at how quiet and powerful it is. It almost made me upgrade but I like not having a cat converter to deal with. Let me know if I can help any more!