I doubt very much that it could be done, it just would'nt be strong enough, you would be better off selling it or swapping with someone who wants a bigger tank. 400 ltrs in an escort van sounds a bit big for the payload too. I'm not sure but reckon the payload for an escort cant be more than 500 kgs or so, i have a ford courier (fiesta derived van) and a 250 ltr tank, I would'nt put a 400 ltr in mine.
Simon