[brucepick]

Yup, I'm with diesel_john in that it's best to block the grill to whatever extent possible, and leave the lower opening to let air in. For exactly the reasons he mentioned.

Unfortunately the truck's front end is shaped a lot like a battering ram, as is my car's original front. Even so, whatever air you keep out of the grill will go off to the sides or over the hood, which is far better than having it bang around inside the engine compartment.

Good going and good luck!