My 67 Dart used to run hotter above 70mph, moded a early Dakota air dam to the lower bumper with the opening in the Dakota air dam help forcing air into the lower part of the rad, no more issues at speed.
Smart boy. Actually the opening is there to reduce wind resistance a little, that's all. What an air dam does under the bumper is push air forward back into the grill and into the radiator. It also helps to block the area between the front of the K frame and the rear of the front valance panel, as well as blocking all the holes and sealing up and cracks and crevices in the radiator support. All of this forces air to go through the one hole it's supposed to and through the radiator.
