Long tube headers help a lot, it is not just about flow as the tuning resonance is way more powerfull but will not show up on a flow bench. There was an article where they tested the horrible stock 273 exhaust on a 300hp crate 360 and then tested the much higher flowing priced like gold 340 "HP" manifolds and you had to squint at the chart to see 5 HP difference but long tube headers that didn't flow much better than the 340 manifolds made 25 more HP. All that said I hate the sound of headers and how they rust out and leak and don't fit so I usually end up with manifolds but I know full well I am giving up significan't power.