With a heavy rig like an H2 you want to be concious of your power curve vs. an ultimate HP number. Having 500 HP is great but if it only makes big power at 6K rpm then it will be useless to you. You will need to keep torque in mind since that is what is ultimately going to get a 6000+lb truck rolling. I would talk to a respected tuner in your area that does engine building, you will likely need a custom spec'd cam and head work that concentrates on low end grunt. You could also look at options such as a maggie charger, the roots blower design is very good for low end power.
Good Luck
