99 Cobra Wheels and Tires Aftermarket?

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 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.