I would be a little cautious as well. I am an accident reconstructionist, and can tell you that you're looking for trouble. I would seriously consider doing one of two things: relocating the tank to the underside of the vehicle, or building a full on roll cage enclosure around the tank. The problem is that should the tank rupture, you've got a fireball directly behind you. If the vapor inside the tank explodes, the pressure wave will blow the glass into the back of your head. With the tank mounted under the verhicle, there's no clear path to the passenger compartment, you've got skid plates and frames protecting the tank, and you won't have the pesky problem of worrying whether something your carrying will puncture the tank when you slam on the brakes. If it were me...I'd probably leave it where it was, but I just want you to be aware of the risks.
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