as fars as chassis goes, the most suggested ones are the 90's impala/caprice. i am working on one mae from a stretched 70 monte carlo grille. don.t know what to tell you about the grille, never tryed an update yet
here's the real thing's grille. You can use bondo to fill the grille, then carve the new grille in with a knife. Not much, but it's a start for you. Or you can glue a whole lotta pieces of plastic together in the shape of the grille. There's other things that need to be filled and reshaped, but the grille is the hard part. I forget who's done it before on this board, but it's hard or else you'd see a lot more people doing it.
Reference pics are the best start, I think this is an 89?