well a lot of those smog era haeds a re the open chamber 74 to 76 cc heads, and the cam manufacturers are spinning the numbers on duration and valve timing to actually increase the dynamic compresion ratio, read higher cylindre pressures to get more power out of the stock heads. If you want a set of good flowing heads for the street your best bet is a set of vortecs or some world cast iron sr torkers. They have the right combustion chamber shape and the intake runners are the right volume and shape to keep the intake air velocity hih at street revs.