I don't know how applicable this is to your engine, but when I recently installed a lower rad hose heater on my 1978 300D I put the nose of the car up on ramps, pulled the lower hose off, drained coolant into a bucket, topped up fluid, started and ran the car still up till the thermo opened and I saw the coolant moving in the rad, lowered car, drove around, allowed car to cool, then topped up coolant with it on the ground.