You will want to have fans mounted directly on the radiator, not distant 2x80mm fans blowing some air on it (though some space is good to kill the dead areas of the fans).
It all depends on the size of your radiator. What size radiator are you using and what fan setup? With that size case, I'd go with a double and mount it lying down on the bottom with fans sucking air from the bottom of the case. If you follow such a design, I would cut out a section of the bottom so that there is room for flow and elevate your case as much as possible.
Perhaps you can mount it vertically under your optical drives and have it bring in fresh air.