I actually dont know how I will do it yet. I will have to make the flares wider so I can get more traction compared to if they where placed under the car with mini tubs. I am going with a concave style Forgeline wheel and want as much concave as possible to get a really aggressive look. I cant really say more right now before I will get started on the project with so big tires.
I dont think there will be any huge problems with fitting 315 tires under there, but it depends on how much you might plan to lower the car. A 315 will maybe also be the limit, you will maybe have to cut some of the flares of so the tires does'nt hit anything when you are driving over bumps and things like that. I hope you understand what I am saying

Are you planning to use custom wheels with a custom backspace or what are your plans? The rear tires on the Mustang in the picture measures 19x11 with a backspace of 5.5". Keep in mind that not all tire Companys tire measures the same in width. Allways go after section widht, that's the total width of the tire when installed!