[thomasrepair]

I just built a 68 GMC with new grille and a new top painted upper grille(both aftermarket). It was a pain!!! luckily i test fitted those before painting, Its not plug and play. I used my die grinder on a couple holes. Long bolts going into the fenders helped get bolts started. I used new cab mounts and new mounts on the core support. I found that I needed to get creative to shim the core support to frame mounts. Most of the parts i assembled on the grille, fenders and cab itself had never seen each other. so i had no reference as to how well they lined up before.
I guess my point is that there are lots of mounting points from the cab forward that have an effect on that grille. Its a process.
I would definitely check all your rubber mounting points to start.
Good luck.