Changing the Water Pump

Use a 27 mm deep socket, I think you can use a short socket but then you will need to attach a short extension to it so that you can then attach your breaker bar.  I suggest that you use a hexagonal head rather than the 12 point.

The nut is torqued quite tightly, my suggestion to crack open the nut is, when the socket and breaker bar is mounted on the nut, place or rest the breaker bar handle on the near-side chassis, i.e. once the socket is on the nut, the handle being at around the six o'clock position, turn your breaker bar handle anticlockwise until it touch the chassis frame.  Ensuring the angles of the breaker bar is square, the socket securely on, very quickly crank your starter momentarily, the turn of the engine would have cracked your nut. You must give this only a quick switch on, crank and immediate switch off to do the job.

Make sure no one is infront of the engine or this set up, If the the tool slips whilst cranking, it could injure some one.  Alternatively, if as in your picture, the radiator is removed you have open access then obviously you may get your air gun on it, you may need to use an inch bore gun if the standard bore is upto undoing your nut.  Make sure the crankshaft is at its TDC position, for when you remove the balance weight, there will be a key in the crankshaft.  You do not want to loose that.  Tighting torque is 400 Nm.