Frontal area is only half the story. You're forgetting CD. I'd say a car based truck is a bit more aerodynamic than your standard pickup truck.
Why?
Because the car based truck has a smaller engine? nope
Because the car based truck has a smaller payload? nope
Because the car based truck has a shorter wheelbase? nope
Because the car based truck only carries midgets, not full size adults? nope
They both are constrained by the requirement that they carry adults so the minimum height and width are pretty much set. The nose of the vehicle can be anything and is not required to be anything other then the styling desired and all the mini trucks are getting more aerodynamic with every year. The vehicle manufacturers are always trying to make every vehicle as aerodynamic as possible but as long as the height and width are comparable and they all have beds, what are you afraid of? Brand new Toyotas are very aerodynamic at the front end. Same with Nissans, VW's, Chevrolets and more. If they are not then they should run slower and shame on you for picking one as your race vehicle.
And after looking at the pictures, I hope they do show up. They would make a hell of a lot more sense than racing a square box. And remember, the class is Mid-Mini truck. What do you think a mini truck is???
But, to address your pictures,
#1, The gray Chevrolet something or other. That thing is an aerodynamic mess. Bring it on.
#2, The silver Ford. thats pretty nice but as with all trucks, the dead zone behind the cab is a disaster but it is better than most trucks.
#3, The silver VW, nice,
but probably not as nice as the Ford. and probably the best of the bunch.
I think you should define what makes a vehicle a mid-mini truck. The fact that it is car based is not a factor (IMO). the fact that it carries at least two occupants, has a bed of a minimum dimension and a payload of a minimum amount seems to be far more relevant.
And another thing, you know what I see when I look at those pictures?
mini trucks.Maybe you should propose a classic mini truck category to protect the vintage iron.