I know this topic has been discussed before but this time I have the cost and problems I encountered. First I will say Ford needs to have a spare that fits the GT with the performance pack, but they don't.
I ordered a 19 x9 performance pack OEM replica wheel from LMR @ $299.00 then a Pirelli 255/40-zr 19-96y @ $227.00 with mounting and balance. then I ordered a scissor jack for the 2017 GT @ $65.00 from an ebay seller. I chose the 19x9 wheel because it will fit front and rear without clearance problems. I know because I did a test fit on front and rear.
In order for the tire to fit in the trunk spare tire area I had to trim the bracket that holds the emergency gas funnel just a bit and it still holds the funnel. Next set the tire pressure to 16 lbs . This will allow the tire to slip into the spare tire well. The tire will not fit flush, its about 3 inches above the trunk floor. I can live with that. the jack is secured beneath the wheel and I also fit a threaded rod that holds the jack and tire in place. The total is $591.00, for me it's better than being stranded on the highway.

 

Good write up, thanks for posting.