The fan shroud kinda eliminates any need for getting the fan real close to the radiator. It forces the air thru the radiator at the same volume and rate as long as the fan is inside the shroud. If you didn't have a shroud then that's a different story. Anything less than about an inch you risk the fan flexing and chewing up the radiator depending on the fan.
The distance from the radiator is not super important if you have a shroud. Then it becomes about tip wash and how far the blades are in/out of the shroud. With no shroud, I have read about an inch clearance to the radiator.