My Boys small block street car runs the heavy iron RHS heads and is flying in the 10's while hearing the radio.
On air travel, I would try the age old trick of duct tape on that windshield trim/moldings and other areas that may be trapping air flow. I suspect the lower rear roof slope/side quarter window area to be suspect on your car.
I've made thin aluminum air dams underneath my cars chassis to help with those cross directional chassis braces. I've even made an air dam between my rear bumpers lower edge closing it off to the chassis. Early B-body guys should take a closer look under there and see that this crucial area acts as a in house designed parachute.
The SSHH guys have been known to paint the under bellies with super high gloss black paint to make things more slick.
Wax the car and the wheels, tires.
The front inner wheel well plastic covers may help you more than you think. I would leave them in for now.