This sounds to me to be a great idea when mounted on a rear rack, provided that the tank you mentioned is a diameter mentioned in the two sizes. It would probably be ideal if you could affix the bracket slung along the top tube or up the seat tube - but I'm not sure your tank would fit either of those places.

My opinion is: that while mounting on a front rack would give you better access to change delivery volume if the tank were mounted there, the weight of the tank may negatively affect your handling, even at moderate speed. You or someone handy could fashion a platform on top of the rear rack with two of these brackets, so you could take a spare tank.

I worry about the change in the bike's center of gravity for all the solutions though. I give care to a younger man (56) on oxygen, and my idea for him was to use a trailer to carry a tank or tanks - this would not change the balance of your bike, only make it a little more work to pedal.