This is a quote from a '57 guy with factory AC. It was from a '63 LeSabre with factory Air and is an after market clutch from Napa.
I noticed the fan was essentially free-wheeling yesterday after I shut the engine off, so I took your good advice and upgraded the clutch from a 1963 Lesabre. The new clutch moves a lot more air at idle. One small difference is that the old clutch has four fine thread studs to mount the fan. The new clutch takes four coarse thread bolts with lock washers.
Pull up this AACA Forum page #7, and thumb through. I think it may be helpful. Dan