I dont know for sure about the grill but since the majority of 67-68 parts are interchangeable, and I dont think Ford made any changes to the dimensions of the grill, I would think the 67 grill could be made to work on a 68 without too much trouble.
But if you have a 68, don't dismiss the front grill too quick, it can be made to look really nice. I painted the backing on mine black and it made the chrome corral and surround look really sharp, IMHO.
I would probably just call a knowledgeable parts vendor like maybe CJ Pony Parts and tell them what you want to do and see what they think. Others on here might have better info for you though.......
Here's mine (just ignore my Anti Let's-look-under-the-hood-of-that-Mustang lock):