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Why don't you show us a pic of the radiator support with the grille removed so we can see what you have going on in there.
Looks like you are missing the grille brackets that bolt to the radiator support. Can't tell from your small pix if the inserts are there for the other holes. https://www.classicindustries.com/shop/all-years/chevrolet/truck/parts/exterior-trim/grill-components/grill-mounting-brackets/
Okay, so click that link and order yourself the brackets, the nylon inserts, screws and you'll be golden