Just want to clarify something. By doing this, your not reducing frontal area. In the picture below everything that is inside the green outline is what your frontal area is. So, if you take off your mirrors you have reduced frontal area. Or, if you chopped a few inches off the top of your cab you reduce frontal area.
What you are doing is taking the area you have, and making it more aerodynamic. This is what most of us focus on since reducing frontal area requires major body work and is usually impractical. IMO you could definitly close up the little slots between the headlights without even thinking about it. However, this is not going to get you far either. They aren't that big, so the improvment is going to be small. I would agree with Peakster about the fog lights, they're already flush with the bumper so you won't see much of a gain there. What you really could see a gain from is blocking the large grill up front, and the slot below the grill. Now, you'll definitly need to a keep an eye on the coolant temps and radiator fan useage, but I bet you could block off a LOT of area before you start seeing the fan kicking in a lot more often. Your truck was designed to have enough cooling capacity while towing/hauling, and normal driving needs a fraction of that capacity. Power is somewhat proportional to heat generation, so if your not using that power for towing you can get away with a much smaller open grill and thus better aero.
