I was in the same sit as you with a warmed over 360 MAgnum, Dyno tests have shown 20-25 horsepower where headers are put on a pretty much stock 340 the better headers (tti) had about 5 horsepower over cheaper ones. Your 318 build will not garner as much horsepower as the 340. The stock 340 manifolds 68-71 will give 11 horsepower over 318 manifolds. With 20 xtra horsepower you are not going to set the world on fire. 20 hp is only 2/10 in the 1/4 mile. If you already have a set of manifolds I would say run them, if not spend 150 bucks and get the Summitt headers.
