Ratchet straps aren't really a problem. It's where you tie them to that's the real question.
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We use lengths of angle steel (about 6cm each way) and fix them in across the van bed with U-bolts made from short lengths of angle under chassis members with two hi-tensile bolts up each side of the member then through the angle in the van bed. This means we only have to drill through the van floor, not the chassis.
The upright of the angle is drilled to accept the strap hooks.
The angle forward of the tank prevents forward movement and the one behind stops backward movement. The ratchet straps prevent the tank lifting off the bed in the event of a crash.
No-ones had a 'full-on' crash yet, but a few near misses and bumps, never had a tank move.