Headers vs 340 manifolds

In my humble opinion....
You will make more power with headers. The OP wanted to know how much. My answer to that is: it's combo dependant, so we CANT answer his question. Also, whether headers are worth it, or desirable, is entirely dependant on the individual.
So: if you want headers, buy em. If you want manifolds, use them. Just know you are giving up SOME power. How much depends on how much motor.
Simple, really.