There's 2 manifold dyno shoot outs floating around 318/340/360/magnum and headers
On a 300 plus hp 360 the difference between 318 and 340 manifolds was like 5 hp.
This type of info is too limited to be of much use, IMHO. This is probably just a peak HP number; no info is given about the effects at lower/middle RPM's, and what the actual limiting factor was for peak HP.
