I'd say that you definately should roll the fenders. Get some 275 DR's for the back. Add drag bags to the rear springs. Headers, an intake and a tune would net you solid mid 12's. On a cool day with good air you might get a lucky break and hit a 12.2, but to consistantly get into the low 12's you have a choice of 4 things. 1) lose weight 2) heads and cam 3) forced induction which will probably get you into the 11's 4) nitrous which will also get you into the 11's.
I've squeaked a couple 12.18's and 12.19's out of my LS2 powered C6. It has 285 tires for more traction, a tune, an intake, and is 500 lbs lighter than the GTO. I'm thinking that you would have a hard time getting rid of 500 lbs, so you are going to need 50 more hp or some combo that has less weight and more power.