Originally Posted by GMC_DUDE

Max. speed would be as if he had 3.90 gears and no overdrive (5.57 * 0.696 = 3.87672). Using stock tires, he would be turning about 4200 RPM at 70 MPH and his max. speed, assuming a redline of 6000 RPM, would be about 100 MPH, about where the stock PCM tune limits the truck.

Not all that bad IMAAHO.

n.b. that many passenger cars in the 1950's and 60's would turn around 3000 RPM or so at highway speeds. It's only been since the common use of overdrive in the mid 1970's that we've seen the now typical cruising RPM's of 2000 or less.
(for the purists yes I know they had overdrive in the 1950's, but it was an option that most cars did not have)