A couple of things that effect performance and may play a role in Kenny's truck.
1) Timing, did John Winslow retard it? If not, then by firing early in the cycle, the engine may develop more torque (argument anyone?), although the engine may also suffer from preignition under the wrong circumstances.
2) Did John change the smoke cam angle? Probably not, which means that there is slightly more fuel delivery, the angle was set differently on the turbo version to reduce smoke.
3) Weight. The M275A2 is only 12,600 lb with winch, giving it about 90 lb/hp compared to 100 or more for cargo deuces.
My HIAB deuce weighs over 17,000 lb empty, but performs like a champ (my multi was also N/A at one time).