In terms of width, there is plenty of room. Just like Jeff, I am running a 285 on an 18x10 rim with no issues.
What you need to remember when you consider rims up to 22 inches is that there won´t be any tire left. The original overall wheel diameter is a little over 26 inch.
So with a 22 inch rim you´ll only have 2 inches of rubber.
My rears are about 27 inches high (285/40/18) but that is as far as I would go. Remember that this will affect your rear axle ratio and reduce performance (although you´ll get better mileage)
I don´t think there is a reasonably sized tire with a low enough sidewall to allow for a rim that is bigger than a 20 for our cars. You´ll have to consider driveability.
Here is a thread I posted a long time ago when I put my big rims on the car. It has plenty of measurements and numbers:
http://www.7173mustangs.com/thread-my-experiences-with-bigger-wheels