There are 2 companys taht already make billet grilles for the Impala, they look like crap IMO.
There are also 2 other choices you have for a different grille, not billet, but better looking IMO. The SLP grille, and the Indy grille. SLP is aftermarket, Indy is from the Indy edition impala, looks kinda like the slp, but still retains the factory cetner chrome strip and bowtie.
Oh and the billet grille manufaturers are streettrends, and Trenz