Those wings on the superbirds/daytonas were very effective. I remember reading an interview with Richard Petty who said it was almost impossible to spin a mopar wing car. The vertical wing supports acted like stabilizers...he said if the car started to come around on you, you could feel the back end get right back in line once it got so far out of shape. He said he never remembered a wing car spinning unless it was hit into a spin.
If you look inside the trunk of a wing car, they have substantial supports inside the trunk to prevent the quarters from being pushed down from all the down force generated. Those cars had lots of little aero tricks...the air extractors on the front fender tops, the A pillar mouldings, flush back windows, etc...not just the wing and noses.
I agree with NZ, at common public road speeds, the effect of a common mopar spoiler is minimal. At race speeds, thats another story.
