John, the further your muffler is from your exhaust manifold, the quieter it will be. That irritating droning noise comes right up into the cab if your muffler is below or even partially below where you're sitting. Because of the spare tire holder, springs, crook to go over the axle, etc a lot of mufflers wind up below the cab, but if you can move it back as far as possible, you'll improve things.
