Aero works like this: oncoming air that gets into the engine compartment, hits undercarriage projections, and which eddys behind the car all create drag, lift, and slows the car; also requires a lot more power to overcome at steady high speeds.
'66-'67? Look at Petty's Cup car towards the end of the '67 season: gaps closed up, sits low at about 5-degree rake, tight/faired bumpers. See also Smokey Yunick's infamous Chevelle and TransAm Camaro.
Keeping the air from getting under the car (air dam) solves a lot of it, because that air hits & bounces around everything.
For a drag car, not enough time is spent at high speed to make all the tricks ET-effective, plus there's weight to consider.
But addressing the nose, reducing grille intake, removing or smoothing things that catch air, all help aero.
A short rear spoiler also helps passing air attach to the trunk lid and cause a tapering effect behind the car, which reduces drag and reduces lift.