Minimum distance for viscous fan needed

I am looking for the room needed from the front of the water pump pulley to the back of the radiator that a viscous fan setup needs. I have had a few members pm me about units for sale
but before I commit I would like to know the space this thing takes up. I have a fixed dimension of 3.5 inches from the water pump pulley face to the back of the radiator. That cannot be changed. How much room is needed for the viscous or clutch fan setup to fit ??