How much will really be gained in this case?
Stock manifold vs ported manifold vs headers?
And at that 6 into 1, or 3-2-1, twin sets of 3, or whatever other variation of headers that could be had?
Not much, headers are an arguable mod on a mild V8's, that's why said the OP the only one that can answer it what does he value more $$$$ or a header on his car with probably very little performance gain.
It really depends on the person for some the cost doesn't matter others it does.