To make a '69 GMC accept a Chevy grille, here's what you'll need to do:
Assuming you're talking about a '69-'72 Chevy grille, you'll need to change the core support and fenders ('69-'72 Chevy fenders have a crease on the leading edge). '71-'72 grilles also do not include blinkers so you'd want to get a '71-'72 Chevy bumper, too.
If you're talking about a '67-'68 Chevy grille, then you'll need to swap out the core support and hood.
GMC Paul knows more about this and can help out with converting the headlight wiring. Be sure to keep your GMC hardware in good shape and sell it to someone who wants to make their Chevy look a little meaner.
