275 wide is about the biggst that will fit on a 8" wide rim but it will perform like ****. Wider is not always better, I would go with the stock size or a 255/45/17 on a 17x8 and no bigger. If you want a wider stance then get some hubcentric spacers or a 10 or 10.5" wide rear wheel.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.