I've got a 300D in my Courier. I did my usual trick of first bolting in two lengths of angle iron across the van bed, one in front and one behind the tank. The tank sits with the curve across the van (ie the front and back walls are the straight ones) I then have two 10ton ratchet straps with the hooks located through holes drilled in the upright web. The straps cross over each other on top of the tank so one goes from the back left to the right front and the other goes the other way. There was a tendency for the straps to slip over the 'shoulder' of the curved top so I made a couple of retaining ties from some scrap metal strip and heavy fencing wire.
I don't fill the tank to the top as I only need about 250l a day, but I've never had any problem with the water shifting while cornering etc.
A point about using ratchet straps: Good quality straps are as strong, if not stronger than steel, and will wrench the van floor and chassis up before they break. If you permanently fit a tank, you need to tell your insurance company and possibly the vehicle registration department. If you use ratchet straps, the tank is just removable cargo, same as any other cargo and doesn't need to be reported.