I would absolutely play with the idea of putting the radiator in the front, as the top intake. Its your best option for cooling really. Just use the top-front spot. Then put another 120mm case fan in the bottom-front as intake to feed your GPU a little fresh air. Exhaust out the back & top.
Remember, if you put it in the back then the radiator will have air blown through it that has already been warmed up as it has traveled through your case. Front mount radiators are the way to go, unless you're covering the entire front and you're not getting any cool air to your GPU. That's where I'd be careful, but you're OK with just a 120mm radiator.
My take anyway.