400ltr tank actually only holds around 350ltrs as some of the volume is occupied by the baffles.
So water = 350kg
Tank = 27Kg (I think or else its 32Kg)
Battery plus poles and other bits and pieces = 50Kg
Driver = 80kg
Total = 510Kg
French vehicles stipulate their payload as not including a full tank of fuel, even if you do include a full tank of diesel (which is less dense than water (relative density of diesel is about 0.

60ltrs = 50kg approx, you are still under your payload, remember as the day progresses you will have less and less water.
Conclusion - well within the payload.