I ordered the same one as dickson, although from a different source. Works great and looks great, and recirculates the coolant back into the radiator so you can run the radiator at full capacity with no air.
It's a tall SS (or aluminum, depending on which one you order) cylinder that mounts right next to your radiator with the existing radiator bolts.
http://www.darkbox.com/mustang/overflow/fan.jpg