Hy 49 4100, V8 conversions are very popular, but they are not an easy swap into this chassis because of the steering box location. A better and easier solution to low power problems would be to install a rebuilt 235 or 261 or rebuild the engine you have. It should be remembered that these trucks were never designed to travel at todays freeway speeds and other systems such as brakes should also be improved if you are going to try and operate the truck in ways it was never designed to operate, hope that helps.