Grille Mounting Issue

[BK'66Step]

Hello! I'm having an issue mounting a stock grill on my truck. '78 Chevy K10 but I bought a '79 grill because I liked the way it looked and it appeared that the mounting was the same. The grill I took off was a rusty steel tube grill that looked just terrible. When I put the stock grill on, I only have the center mounts to screw into but nothing on either side. The radiator core support looks intact, but the mounting is set back at least a couple inches from the center support. Am I missing a piece somewhere? I can't find anything on LMC or Brothers... Thanks for any advice!