Actually headers make a hell of a difference, but if you want a nice bump without the hassle of adding more engine heat and blowing out header gaskets and the head ache install of them, the 340 and magnum manifolds are for ya.
Not trying to be argumentative, but the data from the chart doesn’t seem to show much difference between 340 / 360 manifolds and headers on a 300 hp engine.
At 4500 rpm, the difference looks to be 10-12 hp, about 3-4%. Doesn’t look like much to me.