Sparky, here is my speculation... it does not say the wing has to be over the rear tires, just mounted so the lowest portion is 12 inches above the top of the rear tires.... I think that distance is to prevent someone from using a wing somehow for streamlining.
No wing front or rear can be wider than the inner plane of the tires, just like the body and axle streamlining.
I could be wrong, but if I were to put a wing on I would think well ahead of the rear axle so the down force is pushing the middle, rear and front down.... Of course I try to put most of the weight in the middle to keep the balance close front and back.