Jeff Sparkman pulled these numbers for the '89-91 cars out of the cobwebs: Cd (coefficient of drag) 0.38 and frontal area 23.2 sq. ft.

Someone (who prefers to remain anonymous) later mailed me, and claims to of worked directly with wind tunnel testing of an '89 SHO.  They report the Cd to be .33 and confirmed the frontal area number above.