cloth to leather...
 

I found a complete OEM leather interior for $350.00 on ebay. Would this fit my '13 GT?

Here is the link.. http://www.ebay.com/itm/2012-FORD-MU...b24b5e&vxp=mtr

Assuming it does fit, how hard is it to actually swap?

I posted that then kept looking. It is compatible. So the question is how hard would it be to do?

Remove each seat from the car, take the backs off the fronts, remove head rests and put back together.. Can't be that hard, so a buddy and a case of beer and you'll be good to go.

You might not actually have to remove the seat back, you might be able to make it work by just leaning the front seats all the way forward in order to clasp the seat covers. Take your time and don't get pissed, they are a TIGHT fit and it will probably wear on your patience. I haven't messed with a new mustang though, maybe they're a little easier than early 2000 model Fords.

I did it in a day. Really taking my time to make sure there were no wrinkles. I can't remember exactly why since it's been awhile but I think the seats do have to come out..
You'll need hog rings and hog ring pliers too. I have plenty left if you get in a pinch..
This video is a decent overview:
http://www.youtube.com/watch?v=AGDQC-8byGw

Shouldn't be to hard. I've done recaros and many other seats and as long as you take your time and pay attention to where the hog rings were it should be straight forward.