Also could be the heads and/or block were milled causing the base of the manifold to prevent the gasket on the intake port to not have enough compression. Back in the old days, they made different thickness gaskets (thicker on the intake port pieces) to alleviate this. You may want to remove the intake and put it back together with the gaskets at the front and back removed. Tighten it down slightly and see if the gap between the manifold and block is too small- that will tell you whether the heads and/ or block were milled. If so, i would look for thicker gaskets for the intake ports to allow better sealing or possibly milling the bottom of the manifold.
See this better explained here: http://books.google.com/books?id=Aat...gasket&f=false