My photo is just of the bottom bolt-snap, and it reduces the distance between the bolt-snap hook & the tank from about 3.75 inches to 0.75 inches (on this LARGE bolt-snap). A low-profile d-ring will help some as well. Using the rear-d-ring will also help, because your tank will be negatively buoyant.
Side-mount tends to use sliding d-rings on the waist harness, which is another option, although that means you wouldn't be able to use the waist-harness for much else. They then slide the ring forward as the tank empties, however I'm assuming the pony will remain full as a redundant air supply & therefore a fixed position is plenty good enough.
My interpretation of your bungee-choker on the top was that you were mimicking how side-mount secures tanks, with a bungee that goes under the armpit & is wrapped around the top of the tank. If you're talking about some other method, then I'd recommend looking at how side-mount secures the top of their tanks.