to be honest i am surprised at the amount of people still just "strapping" tanks into vans. remember there is alot of weight there and in the case of an accident you dont want it moving!
on systems we do, all tanks have an angle iron frame surrounding them that is then bolted throught he vehicle floor using 8.8 high tensile bolts and 50mm square plates.
if you get your self to an engineers or welding shop they will be able to make you a frame/cage for not too much.
for the sake of a few quid is it worth taking that risk