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That's exactly how we've done it the few times I've gone in a friend's pickup. Very secure for the tanks, and plenty of room for stuff as you said. On the other hand, it's less convenient than having them on their side at the rear of the bed. Risk vs Reward, sort of.Upright strapped against the bed/against the back of the cab. Safe and leaves more room for othergear.
Here's mine but I live on Bonaire and dive pretty frequently. My wife has problematic wrists so we designed the flap to allow a smooth slide for the tanks and to protect the area between the bed protector and the tailgate cover. My next truck will have the sprayed on liner that covers all of that.