powe mods for an international DT 466

[picknick15]

So I just bought a 2000 International 4700 with a 20' box on it. it has the Dt466 in it with 300,000 miles but was rebuilt 40,000 miles ago. I just got the box off and am shortening the frame for a 12' flat bed and am planning on making a heavy hauler out of it, but the truck is just a little slower than I would like . So what I was wondering is what power mods can you do to these 466 to give them plenty of towing power. I would like to get it up to 800 lbs torque. I know the transmission is geared pretty low so i am not going to be flying but I atleast want to be able to get to to top speed and stay there pretty easily.