Blimey
The number of times this subject comes up :
I always advise that the most important thing is to make sure any fastenings are through a chassis member - through the floor alone is useless.
I use two lengths of heavy angle iron placed across the van in front and behind the tank.
These are drilled in the right place to allow high tensile bolts to bolt them to convenient chassis members - one good fixing at each end of the angle iron.
The upright web of the angle iron can be drilled (again in the most convenient place) to allow fixing of metal straps, caging, ratchet straps or whatever else you want to use.
Doing it this way means you don't have to have a chassis member exactly under the point where you want your straps/caging to be.