Minimum distance for viscous fan needed

I have a post some years ago (search) where I recall giving dimensions for the 273 in my 65 Dart. With the 1" thicker alum water pump, I couldn't fit a clutch fan. I recall having 2.5". I didn't try the Hayden 2765 that others now recommend. 3.5" should be plenty. If I ever revisit, I would try a clutch fan for my 84 M-B 300D since a much thinner fan (most Euro cars) and I recall the pilot holes are the same. I once bolted a 318 clutch fan to the M-B water pump and it bolted up fine (but blades too thick).