If you have the stock manifolds extrude honed they will flow nearly as well as headers. I did this on my stroker 273 and it worked well. Not totally sure about a 340-360 build.
you will not see a difference doing that. waste of money . i talked to a F.A.S.T racer a few years who had it done. didn't do crap for him. he had back pressure gauges throughout the exhaust system too.
