We've mounted tons of speakers behind the grille, in front of the rad. With one large agency we put TWO of the AS124's, side by side, behind the grille of their Crown Vics.... no problems at all.
We frequently put 2 x ES100C's in front of the Tahoe radiators with no issues.
I wouldn't hesitate to mount it the way you are describing. You'll be fine. Those trucks' cooling systems are really over-built for general driving anyway. If you were towing your maximum load up hills in hot weather all day, then it may be another issue.