Re: Headers Vs. HP Manifolds
 

Very possible a prop change may help.

Also, if you do not have trim tabs, adding them will help out tremendously with 'out of the hole' accelration.

Re: Headers Vs. HP Manifolds
 

I just put (2)pair of Stainless Marine manifolds on my 502 MPI's. Nice quality product with a relatively clean install. Definitely notice a feel in the mid-range, but haven't bothered to see if any gain up top.

Not the cheapest on the market, but I thought it was the best value for what you were buying. Trick Marine had excellent pricing on them earlier this year.

Re: Headers Vs. HP Manifolds
 

I re-did my 1989 330hp 454 last winter. Here is the details - link my summary

End result is I went from being able to only pull 4,100rpm with a full tank and 5 passengers to pulling 4,800rpm. I still have yet to do my prop testing to see how much I can bigger I can go.

Let me now if you want any more detailed information about my build. I did the whole thing for under $1,000 - not bad for +700rpm!