If you're talking about the GMC V6, there's little, if anything that can be done to improve the performance. It's basically a low RPM, high torque load-pulling engine, with little or no emphasis on fuel economy or high speed horsepower. For one thing, the rotating assembly is big and heavy, and very suceptible to destroying itself from vibration if strict RPM limits aren't observed.
The machine shop where I ran the connecting rod reconditioning department used to sort of specialize in the big V6's, and even swapping a crankshaft from one block to another, or substituting rods from another engine could create vibration problems. They definitely didn't like any effort to exceed the reccomended top speed. There's a reason the big six in my fire truck has a tiny Stromberg 2-barrel!
Jerry
