Tarnish builds up on the valve stems inside the head where you can not see. Once it gets too thick, the valve does not seat..

For the cost of a gasket, take the head off, pop the valves out, buff the tarnish off, and put it back together.

F.Y.I, any 250 CID head all the way up to 1986 will fit well on your engine, incase you decide to get a replacement head... look around, or ask, folks have them, probably get one for under $100.00