Keep in mind, folks, that not only will these discs have better stopping power than the drums that will be replaced, but your truck engine and drivetrain will also have an easier time of rotating the rear wheels.
Instead of having to spin up 50 pounds worth of brake drum (2 each 11 inch drums at 25 lbs apiece), they'll will only have to spin up about 30 pounds worth of brake disc. According to my college physics-based estimate, that's worth a gain of about 0.5 MPH for a 1/4 mile run, for a perfectly stock engine. That translates into a gain of about 5 HP, which isn't much by itself. However, this gain is on top of the gain realized by reducing unsprung mass, and the gain from having disc brakes over drum brakes.