headers vs manifolds on a 318 2bbl difference ????

That's why care is required watching the videos. In the engine masters video 2.5" exhaust vs 3" exhaust,
they proved the 3 1/2" was superior on a 600hp big block by about 13hp!
But under 3500 rpm the 2 1/2" exhaust was noticeably stronger...and they skipped over that so fast i'm betting
most missed it, but they did point it out. more power up to 3500 rpm is important in a street machine.