The cooling issues are very unique to the HMMWV, it was designed for low speed operation, the factory fan moves allot more air than any electric fan/s that are available.
That hole in the hood is a big issue with me, we learned back in the 60s (and maybe actually earlier) that a hood scoop turned around backwards worked much better at highway speeds as the air at the front half of the hood is very disturbed or there is no flow at all. (also known as cowl induction)
Look at the Funny Car, that is the one race car with what could be described as a factory body and the induction is at the rear of the hood.
One of these days I will get off my lazy butt and put a bunch of tufts on the hood and prove in, I have watched various insects just bounce around on the hood next to the opening whereas if there were a significant amount of air moving at that point it would have blown the insect off.