Drag cars don't spend much time at high speed, but aero improvements would help above 60 or so. Air going through the car increases drag as opposed to air smoothly going around it. Getting the nose down and addressing airflow at the roof/windshield and under the car would be your biggest improvement; hard to do anything about the roof/windshield without cutting. Back in the AFX days, it was learned that a spoiler on the trunk lid brought MPH up. The Dart's concave rear window hurts aero. I remember a Car and Driver study on a Dodge B100 van where the biggest improvement was removing the mirrors.