RockAuto used to be a gold mine for info like this. But at some point things changed and now it is much more difficult to find the pertinent info. Seems like Raybestos is one that gives some info though.
Looks like the 2012 GT rotor is 1.89" in overall depth.
More Information for RAYBESTOS 680811R
Maybe the 2015+ Brembo equipped Mustang rotor?
More Information for RAYBESTOS 681954R
That gives you about .6" more clearance, but they are 15" rotors. No idea if you could fit an 18" wheel over that, but kind of doubt it. I know the Mustang guys have threads about mounting 18" winter wheels on cars with the Brembo brakes, but doesn't seem possible to me. I guess you could see how much room you have in your rim though.
Might look at the 4 piston Brembo rotor from a Challenger.
More Information for RAYBESTOS 780690R
Those are 14.2" in diameter but only give you about 3/8" more clearance to your wheel spokes.
How about a front rotor off an 2010 MB S550?
More Information for RAYBESTOS 980743R
Only 13.8" in diameter, but better than 5/8" more clearance to the front of your wheel. But will probably take a machining step to clear the wheel studs and hub.
Not sure how much clearance you need to get your calipers to clear.
Keep us posted.