http://www.etrailer.com/Radiator-Fan...le/D16825.html
This is what I have mounted on an aluminum (2) 1" core radiator. Haven't got it on the road yet so I can't say how it works.
I spoke with the guys at Ron Davis who gave me the lowdown. If you are going to hook anything to your bumper you must have a mechanical fan / shroud and preferably a clutch.