strap the tank down to the factory fitted anchor points, which rachet straps, change these every year and you will be fine.
I have a 650 tank and use 4 rachet straps that have a load of 800 kilos each, and when I did crash my van, the tank did not move a millimeter.
The other problem that a lot of people forget is the bulkhead, if you don't have one and you crash, a small piece of equipment that weighs a couple of pound is going to cause some serious damage to the back of your head, when it smacks into it at 30 or 40 mph.
and in my crash, my hose reel took a trip around the back of the van, and caused some impressives dents in the walls of the van.
Amazing. During the crash, I bet you felt like you were up in space with stuff flying around the van.
I never noticed a thing, I was enclosed in my drivers cab, I was a bit concerned about the wall I was heading for, because on the other side, there was a thousand foot drop off a mountain.

But when I went to the recovery yard to strip my system down after the insurance company wrote my van off, I was amazed at the damage to the inside cargo area, and even more impressive, was my metal solid hose reel up bent up and twisted.
I am all for safety, but i am very suspious of any company that goes on about it, yet will fit their system to a van without a fitted bulkhead.
If they are that concerned about safety, then they should say that they will not sell their systems to anyone who does not have a fully protected driving cab.
the other points to look at are the crashes they are conducting, most seem to be front on collisions, what about side and rear on's.