If it's the 2.9 then an 850l is probably the biggest you should go for, it's pushing it though...
Rog, the reason you feel the water sloshing about is cos you need more baffles in the tank, easy enough and cheap enough to do if you don't like the feel of the sloshing water.
One way is to fit downpipes, cut down and slotted in like a honeycomb, but that is pricey.
The other way is to use land drainage pipes, way, way cheaper, they are flexible too, already have holes dotted throughout them so you should be able to feed it through as you please...to a point anyway.
I keep meaning to do it myself, but never seem to get around to it
I've the same van and have a 650l tank, I can feel the water in there when the tank is half empty, but the effect is minimal, even under very hard braking.
Ian