Nice truck My bro has a 63 fleetside, nice ride.
anyway the whole point of a fixed fan is that it bolts to a spacer between the water pump pulley and the blade.
otherwise you run a clutch type fan and go from there.
The whole purpose of a fixed blade is to pull more air at lower rpm constantly all the time, at higher rpm load is reduced as the blades flex out of the way and the air flow from the speed takes over.
Always was one or the other far as I know.
Mr. Gasket makes a spacer for those fans available in almost any length you should need.