check your existing exhaust header tube length and diameter and the collector dimensions,
theres a good bit of power lost or gained by matching header dimensions to what the engines the cam duration,
lsa and engine displacement and engine compression ratio and ignition timing and fuel/air ratio want as the ideal compromise
i have very often seen guys just order headers that fit the car and engine without doing the required math, remember commercially available headers are in most cases designed to fit and function and be used on as many different cars with similar engines as they can , at the lowest cost to produce, this almost always results in commercially produced headers with smaller diameter and shorter tube length's and short & badly designed collectors yes it takes some effort and research but its very common for well designed custom headers to broaden the power band and quite often you can gain an extra 20-40 hp over off the shelf headers I have seen a few cases where custom headers and collectors made a very noticeable increase in the power and resulted in a noticeably broader power band and over 50 hp gains on some engines I built, remember properly built headers cost more and in about 70% or more cars the owner never considers that off the shelf headers are not even close to ideal.
in fact its one of the reasons I got into TIG welding ,I could never find off the shelf headers that were even vaguely close to the math predicted ideal dimensions, measure your existing headers and then do the required math and remember primary tubes that are too small or two short or a collector that's too short or the wrong diameter can really narrow your torque peak and the power band and limit the rpm potential.
id also point out that the correct headers almost always require retuning the fuel/air ratio and might require a change in the ignition advance curve to produce power you never had access to before the header upgrade especially if your cam has a fairly tight LSA in the 104-108 range where the headers really respond to high compression as the peak power might really surprise you once the proper headers allow the engine to scavenge the cylinders correctly as it generally requires a richer f/a ratio, many commercial off the shelf headers can be 4-8 inches shorter on the primary tube length and noticeably smaller in diameter than the match suggests is ideal and its almost certain the collector dimensions were designed for easy connection to a stock exhaust with zero thought to power potential
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