What is the correct fan to radiator clearance?

That is a very short fan clutch, like the one in my 1985 M-B 300D. For those with the opposite problem of no clearance, the M-B fan bolts to the water pump (same pilot hole diameter), plus the nylon fan is thinner. Many rodders use a 1990's Jaguar fan-clutch for tight clearance. So little clearance to the hub that owners have to ground one side of a box wrench to get it over the bolt head. In my M-B, I switched to bolts with a larger head so I can use a box wrench without fear of rounding off the head.

I would get the standard clutch to move the fan more into the shroud, for better airflow, easier access to the bolts, and less chance of whacking a finger while fussing around with engine running (and use clip-on ties). I understand the main risk of the fan being too close to the radiator is if you drive thru a deep puddle, the water can bend the blades forward to mangle the radiator. The Car Talk show mentioned that.