Checked the chart on the D-port heads after '67, 17% difference on the intake and 24% on the exhaust - worth doing a bit of selecting. Looks like the '70's heads (5C, 6X) are not as good on the exhaust as the earlier ones. But as they say, the tests show general trends and individual heads will test differently.

Of course a good porter should be able to even those numbers out and add another 15-20% without too much cost.