'D' heads were specific to only the '68 and '69 W30 and H/O's. These heads flowed almost as well as the 'F' heads on the 70 W30. I will refer you to the chart on head flows that my buddy has put together,
MidAm Performance
As for the '68 motor, they differed by having an a/c motor and a non-a/c motor. The a/c motor used a milder cam and 'C' casting heads while the non-air car received the 'D' heads and the more agressive cam. The '68 used the standard 442 manifold.