I have my 350ltr flat tank in the cback of a astra van, I have a bar welded from one wheel arch to th eother one to hold it back, its buted up aginst a metal bulk head, and has a bar either side to stop sideways action. Pretty simple actually.
its not even fixed to the bars, just held in place by them, yeah I got a few metal garage protector bars and cut them to size, and had them welded for a tenner. How is the fuel tank, any pics of the load area of your van?If you have the bars behind drivers seat, could you not strap it onto that?