I've just put a 300lt "D" tank in a Courier. There weren't any convenient chassis members so I bolted in a pair of 50mmx50mmx5mm angle irons - long enough to reach across the van and bolt through to the chassis. Then I drilled the angle irons in the right places to attach the tank restraints. With 12mm high tensile bolts, that lot is so strong it will destroy the van before it gives way.