How close is to close

I have a clutch fan that I am trying to use on my 70 Duster 340. It is a 20" fan off a 86 Dodge Ram. The clutch is about 3 .25 " thick. With it all mocked up on the car the clutch itself is 1/2" off the core. The blades are 1" off the core and about 1/2" off the tanks. Is this to close? I could get the Hayden 2765 clutch that is shorter but I have read they may not be strong enough for a steel mopar fan. Any thoughts or input would be great.
Thanks Mike.