The ports on 318 heads have very inconsistent castings from port to port on the same head. I scrapped two dozen heads for this reason. I finely found apair off a truck that look decent as cast. The 318 aftermarket manifolds do have aport mismatch on 318 heads. The 273 heads are the best bet for small port small valve heads, and they are closed chamber. The valley pan I refered to fits under the intake and keeps hot oil off the bottom of the intake manifold.