Factory Gt500 is 265/40/19 and 285/35/20. I have SVTPP wheels on my GT and I just put new Michelin PSS on it. Only I went with a 255 in front due to the price difference. A 295 would work fine in the rear also.
I would run 10" rims all around with those tire sizes. But the staggered 19/20" looks better than all 19's or all 20's in my opinion. Plus go with a 40 up front and 35 ratio in the rear, will look better and ride better.
I'm looking at either that combo on 19x10s and 20x10's or 19x10's all the way around with 275/35 up front and 305/30 out back. I liked the 275/315 of my previous mustangs so like the idea.
Not sure how the 305/30 tire being .3" shorter than a 275/35 will effect Traction Control, though.
leaning towards 19x10's with 2013 GT500's, since i couldnt decide between gunmetal and black I'm think blacking with a metallic gray pinstripe, I photshopped something to this effect. (My 5.0 is Sterling Gray Metallic)
I have the Ford Racing Performance Pack 10" rims with 265/40/19's up front and there is plenty of room all around. I did need to use an 1/8" wheel spacer so the Brembo caliper would clear properly.