I use Michelin X-Ice Xi2 in a 235/55/17 OEM size on my OEM "bullitt" wheels. I think they made them in a 225 size, but I didn't want the car to feel too squirmy on dry roads.
I agree with the fellow who said you want the skinniest, tallest tire you can get. Most of the time a good rule of thumb is to get the OEM wheel setup from the base model of your car. The Mustang does have a V6 model with 16" wheels, but a 16" wheel won't clear a GT brake. So you're stuck with 17s as your "smallest" wheel option.
You must have 4 snow tires for the concept to work correctly. 100% different driving experience. Your car actually drives in the snow.