Reading between the lines it seems to me that MIRA accept that any crash at speeds in excess of 30mph will result in total destruction of the vehicle and non-preventable injury/death of persons in the path of the dislodged cargo.
They therefore seem to be concentrating on minimising the effect of crashes up to 30mph which presumably they regard as survivable.
In other words, don't worry about crashing at over 30mph, you're probably going to be dead.
Up to 30mph if your tank etc is secure, you might survive. BUT IT STILL BOILS DOWN TO "HOW WELL IS YOUR TANK SECURED TO THE CHASSIS?"
If the tank and chassis are in effect one solid unit, then the tank is safe but only up to the point where the chassis fails. So in reality the limiting factor is the quality and strength of the chassis.
We realised this a long time ago and so designed our fixings using angle steel across the vehicle cargo bed and secured with 12mm HT bolts passing down each side of the 'fore and aft' main chassis members with a short length of angle steel across the underside of the member to form a "U" bolt around the chassis. With the upright web of the angle steel drilled to accept the claws of as many 5ton ratchet straps as we can fit in, we believe this to achieve the 'one solid unit' as above and therefore as safe as it is possible to be.
Having read Alex's report that the main movement of the tank is likely to be horizontally forward, we now also incorporate a further angle steel braced across the front of the cargo space and bolted to the vertical members forming the support around the side and roof of the driver's cab. This steel is positioned so that it bears against the front of the tank approximately half way up.
As the materials we use are far thicker and stronger than any of the chassis, then the only way this can fail is when the limit of the chassis is exceeded and the vehicle is destroyed.
Obviously we haven't had this crash tested but we have complete confidence in it within the limits of the strength of the vehicle itself.