I've done a bunch of experimenting with back pressure on motorcycles. A smaller diameter pipe = better performance at low and moderate RPM's, and better fuel economy. A larger diameter = more high-demand, high-speed power, but less economy and more backfiring on deceleration, and needs a richer air-fuel mixture to keep from running too hot. I think a 2" pipe and a low-restriction muffler will give a Stovebolt its best "happy medium" in a light truck.
