2004 4300 26' van truck

I recently bought a 2004 ( don't ask me if its a 2004 or 2004-1/2, I have no idea how you tell, yet) DT466E, allison auto, 2' morgan dry van, liftgate. The truck has 327,000 miles, feels good lightly loaded. Put a "max load in it and it takes it about 4 miles to make it to 55 mph, and forget 65 unless you are going downhill. I changed the oil, put on a new fuel filter. I read somewhere that DT466's have a "rock catcher" under the fuel filter. I did not really see anything and was hoping to find a manual page or diagram of that.
The truck also seems to "miss" when heavily loaded, it just does not seem as smooth as it should be. It is kind of there empty but not nearly as pronounced.
I would LOVE to have more power. This thing started life as a Ryder Truck. The valvecover says 190 hp. I have read the 466e's have limited "turn up" but there are a couple places near me that claim " big rig tuning ". I have also read my first stop maybe shoudl be an IH dealer and see what its turned down to.
Whats your experience squeezing some more power out of this vintage dt466's?