I had a problem with header gaskets sealing on our small block el Camino. I got to looking at one of the gaskets I took off. The bolt holes appear to be pierced in a separate operation to that of the port openings. The holes are so far off centre on the one end that they dropped the gasket so far that it was almost off the raised area of the header flange that seals the gasket to the head. Something to check before installing your next set of gaskets.