Well as it turns out you can have yer cake and eat it too. Read
this and tell me if it does not say that you reduce lift by a lot, and reduce drag by a lot, together in one easy step. Just build a cap with an 8° slope. See page 85, then branch out from there.
This 7° cap would just help reduce the fairly large lift that is inherent to the design of a pickup, making it essentially aero neutral.
If you want to create some down force (a Negative Coefficient of Lift -Cl) without a bunch of drag, yer going to need to add something like this I suspect.
And you get this. Dunno how legal it would be.
You could take off the "Stinger" and have a real fuel saver here.
Now I imagine other factors would creep into the equation, such as just how powerful an engine are we talking here? Where do you risk breaking traction? How fast do you end up going at half track and end of run for either aero efficiency, or downforce to play a critical role? All these things would have to be fairly understood I think before deciding what to do.