1970 340 heads on a 73 318

If you are running factory manifolds, with no plans to change even to HP manifolds (the 68-71 340 HP manifolds..),

Even if you run the "HP" manifolds you would only most likely gain a whopping 3-4 HP over your 318 manifolds(even after a cam swap). So a step above stock cam would be my suggestion becuase without going with a bigger cam,headers,something like a RPM manifold so if your sticking with the manifolds jsut pick a bit bigger/stronger cam to "wake it up" a bit.