Installing headers, or more carburetion on an otherwise stock vehicle without also doing major modifications to the cylinder head, such as porting, polishing, and/or installing bigger valves is like joining two pieces of log chain with a paper clip. The stock GMC exhaust manifold is capable of handling all the volume a stock cylinder head can flow, and then some. The only other reason to install headers would be to impress the mouth breathers at a "show and shine".
Assuming the engine builder has done his homework properly and spent a bushel of money in the process, short headers increase top end horsepower. Long ones boost low end torque. The selection of the camshaft and a bunch of other nitpicking details will have a huge effect on header length and other modifications. Either one is a compromise that lowers, not raises the numbers an original equipment manifold will make over the full range of performance. The builder will be trading off performance at one end of the spectrum to increase the other. The guys who designed the engine didn't fall off the turnip truck and end up in a research lab by accident.
Jerry
