James:
In your novel Patriots, you describe a 1968 Ford Bronco getting its radiator shot out. The only reason that the radiator was put in the front of early cars was because they did not have effective water pumps in the beginning, water flowed through and was cooled. 90% of the air that cools the radiator comes from under the bumper. You can totally block off the upper portion without any overheating issues. So a series of slats if you do not trust it, could be welded behind the grill if you wanted to.
If you are still afraid of overheating, a trick I learned years ago is very effective, and that is to add another windshield washer tank and pump, with the sprayer aiming at the front of the radiator. You then get evaporative cooling that is so effective you have to see it to believe it.
I would think that your mechanic in the book would certainly have foreseen that weak point and taken care of it. A metal plate could be welded at an angle from the bumper to the top of the radiator. It would then have functioned fine to bounce bullets up out of the way. Granted, weight saving is important, so you again could have used Lexan for half the weight, I would have also reinforced the floor, firewall and sides with either Lexan or Kevlar laid up as fiberglass panels. – Steve D.