The header primary length and the collector length have a bigger affect on what RPM that peak power is achieved rather than how much. Shorter primary tubes will result in horsepower and torque peaks that come at a higher RPM. Longer primary tubes will result in horsepower and torqe peaks that come at a lower RPM. There probably won't be much of a difference in the peak number, just at what RPM it occurs at. In a truck, you would probably be much better off with full length headers.
If you really want to know alot more about this, go to
http://www.headerdesign.com/ and start reading. There is also a free calculator that will calculate the optimum primary size and length, and the optimum collector size and length. You will probably be surprised to find out that most headers out there aren't designed very efficiently.