It really comes down to "usable" power. Will the egnine be a small block or a big block? Naturally aspirated or forced induction? It is possible to build a small block that will make 800 HP, but driving it on the street would not be a pleasant experience, especially in a heavy vehicle, since it would have a torque curve that is very narrow and doesn't even start until 3,000+ RPM. /forums/images/graemlins/frown.gif Great stuff in a lightweight drag car that's set up for the engine. But not the hot ticket for getting 7,000 lbs of truck moving in heavy traffic.
If you want massive torque, there isn't much that will beat a hot rodded turbo diesel. A Cummins or Power Stroke can easily be tweaked to make 700 lb/ft of torque, and at under 3,000 RPM! /forums/images/graemlins/shocked.gif You can spend more cash and get them into the range of 1,000+ lb/ft of torque, but at that point you really have to start looking for a massive drivetrain to live behind it.