If you look at the dimensions, the 500 or 350l 'flat' tanks are not actually that flat. My 500l is 62cm tall, whereas an upright 350l is 75cm tall, so not a lot of difference. The 400l is a different thing altogether, the flat version being only 36cm tall. I have not used a 400l, but i would imagine with a flat one, you would have to park carefully, especially towards the end of the day, to make sure the water was going to be sucked out of the outlet!.
There are some guys on here who use upright 400l tanks, this would seem the best solution, but i would feel pretty vulnerable up front, with 400kg of water right behind my head ready to flatten me in the event of an accident. If i had absolute confidence that it could be restrained, then i might consider it myself, but none of the installations i have seen through photo's on here have looked very safe.
For your van, i would be looking at a 350l or 500l flat tank, pushed up against the crossmember at the front of the load bay, combined with straps to stop rearward or side to side movement. In an accident, that combination means there is no chance of the tank moving forwards. For that very reason, there is a good chance my next van will be an expert, or its fiat and citroen cousins.
As for water movement. When i fitted the 500l tank in my connect, i assumed the main danger would be side to side movement, so fitted the tank with the baffle in line. I dont know how much difference it makes with regards to the side to side movement, but when i first started using it, i could really feel the water moving forwards and backwards. It was very unsettling when i came to a stop, and when accelerating!. I stopped noticing it after a few weeks, but i don't know whether or not this is due to my driving smoother, or just getting used to it!.