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.
Id like to see some flow data from the honer on that one before I would pony up the bucks. Headers use the exhaust pulses to scavenge the other pipes in an orchestrated dance, its not just square inches of exhaust (which the headers have 4 fold over any manifold). I know my 65's driver has a factory steering shaft pinch and a mean 90 bend at the end, although not as bad as the 383-A body ones. a 416 is 65% larger than a 273 and the manifolds are not that different between the 273/318 and the 360.
