I came across a FR500C Mustang developed by Hajek Motorsports for a Bonneville speed record. While this is certainly no fuel efficiency wonder it does have significant aerodynamic modifications for that high speed run.
Plenty of photos to look through here:
Bonneville 2008
and here:
Wind Tunnel 2008
slick 'fast cut' video here:
Unfortunately, I haven't found any more info on the wind tunnel and aerodynamic work on the car. The website is more focused on the motor than the aero
FWIW,
Tim