Greg, I think you'll need to extend the cabin and chassis of your Elephant. The German large calibre guns used a system of inclined slides to absorb a lot of the recoil when they fired. It made the carriages larger but lighter, compared to most other artillery pieces of similar calibre. Essentially the gun mount had some recoil absorbing accumulators on it which absorbed about 50% of the recoil and that was in turn mounted on an inclined slide with it's own recoil system, that absorbed about another 30% and the remaining 20% was absorbed by the carriage resting on the ground. That meant a long carriage was required. So, either you'd need to build bigger accumulators onto the mount or give it a very big recoil spade (and accept a decreased mounting life).