I'm running 17's on my 65 Cuda, 275 40R17 in the rear and it fits, wheel wells are all stock with the springs moved in and the rear end set back 3/8's of an inch to centre the wheel in the wheel well. I'm running an 8 3/4 with wilwoods disc's on the rear.
The trick seems to be to get the right offset, my rear rims are +24mm with a 5.95 back space. On the front I have 225 45 17's., with wilwwod Disc brakes on the 10" drum spindle. Rear rims are 17 X 9, and front 17 X 8. And yes I have a Percy's fitment tool, and it really helped figure the offset out.
This were purchased from American muscle, yea a Mustang place and found to be very helpful when ordering.