I've debated this myself whether to mess with headers. Andy made 550 HP w his 470 with HP manifolds/3" mandrel pipes. Way back "Headers by Ed" sold header kits and some custom headers and they quoted an independant study by Hot rod iirc that found a great amount of gain by utilizing equal length pri pipes and that was their big selling point and that's the only thing pushing me toward considering headers as opposed to the HP manifolds or maybe the max wedge manifolds which are even better and have a 3" flange outlet. The BB logs I wouldn't even consider