My vehicle is an 07 GMC Sierra Classic P/U, 4.3L engine, and a 5 speed manual trans. I like information on sources to purchase a set of headers, opinions, and most of all experiences of those who have added headers to similar vehicles.
For normal use stick with stock manifolds, they’re hard to beat for low end torque, which is what you need in full-sized truck with a V6.
If you have warm-up catalytic converters, those are just off the manifold, you can’t put on headers if you live in an area that does smog checks.
Headers will provide you with little more than noise, heat and exhaust leaks. A set of dual exhausts, while no BIG improvement, will do more for mileage and performance that headers will…
Headers are only going to make a noticeable difference if used in conjunction with other items; full exhaust work, intake work, and preferably camshaft and cylinder head port work.
If you do go to headers I would strongly advise staying away from Flow Tech headers, a division of Holley. I’ve had the dubious honor of wrestling a couple sets of these greasy pigs and you can pretty much figure on problems with them; lousy welds, uneven flanges, etc.
You’ve got to like headers in which the weld bead protrudes into the gasket surface, bolt holes have to be wallowed out, etc. 