most of the truck engines are different from the lesser weight chassis, but if the bell is the same, you can swap it in. The truck engines usually have a 4th ring, and the water pump setup may be different, and the bell may be different, you will need to check all that out. The heavier chassis used a round bell, to keep them universal for mounting different trannys to, where as the lighter automotive used a more trapezoid type bell. I have seen it go both ways, just have to see what you have. Best way to make power is with more gears, but it depends what kind of money you have to play with. A good towing cam and a few mods can bring in some power too, just depends how you want to go. I don't know why, but the aftermaket almost always can provide better cams and such, I would think the OEM would be best, as they design the thing, but maybe it's the big oil conspiracy...