Sorry, but i think you would have to be insane to fit a 400l tank upright, with dimensions like that. How would you stop it killing you in an accident?. If you are worried about security, fit a bulkhead. A flat 350 would be much safer, but if you are worried that 350 is not enough, and a flat 400 is not practical, fit 2 upright 210 tanks side by side and link them.