I would go 275 in the back. Other posts have once said that 315's will rub the IRS bolt unless you shave it or replace it with a smooth one. Ask, Uncle Meat. I think he posted about this once.
I'm running 245 40's on the rear with 18x9 rims and there's plenty of room to go wider. I'm wishing I had some 18x10's on the rear with at least 275's.
I ran 18x9 saleens on my 99 cobra, up front I believe I had 255's no rub and out back was either 285 or 295 with no rub, but keep in mind they were nittos and they run smaller.