Headers vs 340 manifolds

We all know that changing just one thing on a stock engine usually results in only a small improvement (intake, carb, cam, headers). I think a more interesting question is on a stock engine where is the biggest bottleneck limiting the making of more power or is it a waste of time only doing one thing? If you could only upgrade one component where is the money best spent or does it matter?

I know its 3x the $$ over the other upgrades but perhaps bolting on aluminum heads eliminates the biggest bottleneck (probably been tested already....I didn't search). I might have tried that with my latest stock build had price and availability gotten crazy.