Can of worms, this
I would never trust any brackets/cages bought 'off the shelf'.
In any installation the paramount consideration is how it's anchored to the chassis.
I've installed 7 systems so far, and after a lot of thought and experiment I now believe the safest way is to fit heavy duty high tensile 'U' bolts around convenient chassis members with the threaded ends poking up through the van floor. Inside the van I fit two lengths of 5mmx60mm (or similar) angle iron across the van bed, one at the front and one at the back of the tank. These are drilled to accept the ends of the 'U' bolts and secured with locking nuts. The upright bit of the angle iron is drilled to accept the claws of some 10ton ratchet straps which fit over the tank.
If you can't get 'U' bolts then 2 12mm high tensile bolts either side of the chassis members with a length of 10mm plate under the chassis member with each end drilled and the bolts passed through it either side of the chassis member.
In the event of a crash, the critical part is how well the whole lot is attached to the chassis - bolt just through the floor and it will rip up like tissue paper.
With the above arrangement the entire vehicle structure would have to give way before the tank comes loose - if the crash is serious enough for that to happen, you're dead anyway
A major advantage of this method is the insurance 'modification' rules. The tank is just cargo, held by restraining straps and so is not a permanent fixed modification.
You can buy all sorts of 'cages' and 'brackets' but remember - if the tank isn't held in place by solid, heavy duty fixings around the chassis, it's of no use whatsoever. In the event of a crash, the whole lot will come flying forward into the back of your head.