Hi Dean, isn't it funny that not one of the repliers have asked what size tank you are refering about?
Plenty of silly advise when they don't know the size of tank and that will provide you with the answer. If its a small 300/400lt tank use the vans pre assembled points (if applicable) if not available have them made by a local fabrication company with extra strong steel.
If over 400lt then get a local fabrication company to provide bolting points through the chassis this will give you extra secure points, they will also provide a tank chassis frame cheaply but provides secure fixtures that will cope with an "accident" incident.
Cheap = no safety
Local Fabrication companies are brilliant, they can build you what you want at a good price.