It looks from your pictures that the big truck grille is just shorter (less high) than the smaller truck grilles. Possibly the vertical spacing between the bars is different (larger on the big truck grilles.) So grille bars are likely interchangeable. Yours look to be in great shape, so what you'd need are the end pieces and center vertical supports from a donor grille and swap over your bars to those. Shouldn't be too hard to find a grille with trashed bars where the vertical supports are OK. That or a decent complete grille.
