Water Pump, Vibration Dampener Removal

[BDBENZ]

I could use some information on the vibration dampener removal. I am replacing the water pump on my 87 560 SL. To gain access to the bottom bolt, I understand that I must remove the dampener and drive pulleys.

My first concern is the torque of the nut, it is listed at 400nm, and will I be able to get this thing loose with an impact gun? The impact gun specs at 700nm in reverse and 650 in forward. The second concern is the woodruff key on the crank. I have marked the position of the balancer to the block and also the pulleys position. When the balancer is removed, is there risk of the crank being in position where the woodruff key is on the bottom and could it fall inside the timing case cover? I am hoping that the key position on the crankshaft is external of the engine covers. My manual is not specific on the location. Please, could use the advice, I would like to try to cover my bases.

Thanks,

BDBENZ