Ratchet straps would easily hold a 400 litre tank, you can get them with a breaking strain of 10,000kg (
www.northerntooluk.com)so well within their capabilities.
The important point (as you ask) is are your anchor points strong enough and are they in the right place to allow the ratchets to secure the load. Ring Ford or look in your handbook to find out what they are rated to. If they are not up to the job then as 'windowwashers' has said get a welder to weld strengthening plates and loops in exactly the places you need them to be. To be safe you would need 2 straps front to back on the tank and 2 straps side to side, which means that you would need 8 anchor points.