The closed chamber is a good starting point for a quench engine. If the flat of the piston comes to about .040" away from the flat of the head, the turbulance prevents detonation and lets you run higher CR.

The 516's would not originally have had hardened valve seats and the flow wasn't the best of iron heads. Mods you describe to the heads may address these isssues. If they are in good shape, they would work well on a street or moderate performance build. If they need a lot of work, you'd probably be better off with Edelbrock RPM or 440 Source heads from both a cost and performance standpoint.