How much was block decked, is it square. Most older heads have been milled at some point. Are the heads square, did you cc them. How thin is the valve cover surface on intake side. Many heads have been milled on the intake and if it was not levelled properly you could have that problem. You can do the manifold, but that becomes a one off for that combo only. I would think you are better off finding out if the block or heads are the issue and getting them square.