I'm using 275's and 315's on my FFR MKIII, but like Joe mentioned it also depends on your wheel/offset. Of course I also used the IRS not the stock Mustang rear. I did have some rubbing so I bought 1" spacers and moved the rears out a little. I really like the way the fill the space in the rear, but I will probably go with 245's in the front when the time comes because they do rub a little.
I live right over in Carrollton so if you'd like to swing by some time and take a look at my setup drop me a pm.