1968 Chevy Impala fan

If you haven't change the thermostat-do so and see what happens

When the fan clutch is working properly it will freewheel after a shutdown for a few hours-when engine is at normal temperature it will resist a little when spin by hand

Could be air lock in the cooling system-try bleeding it

Always bleed air from cooling system after replacing coolant. Set heater for maximum heat. Remove radiator cap. Loosen drain plug and remove drain bolt (if equipped) from engine block. Drain coolant reservoir. Fill coolant reservoir to MAX mark with 50/50 water-coolant mixture. Loosen bleed bolt and fill radiator up to base of filler neck. Close bleed bolt when coolant flows out without bubbles. Tighten bleed bolt. With radiator cap removed, start and operate engine to normal operating temperature. Add coolant if necessary and check for leaks.