[gboone]

here are the real CD figures

This is not a guess, this is published data:
1979 0.39
1975 - 78 0.43

Keep in mind the coefficient of drag is a ratio of the drag compared to a flat plate of the same frontal area. The frontal area of the early Z's is small compared to many modern cars. So for example purposes, a Z car with 20 square feet (just a guess) of frontal area with a CD of .39 will have less wind drag than another car with a CD of 0.35 and 25 feet of frontal area.