those bolts are indeed 3/8"-16 x [approximately] 1" long.
i have the nose of my charger [grille/bumper/valance] off at this time, and just looked. these parts have not been off this car since new, and i have owned it since 1970.
most holes are larger than necessary to provide some adjustment, but mostly for ease of assembly.
i can take a pic or two if you want.