Increasing world gen height to 256 - a simple but significant problem.

Hello everyone. I have recently been experimenting with raising the maximum height of terrain generation in my mod's new dimension, to allow the generation of much higher mountains. So far this has gone fairly successfully except for one problem:

In doubling the max height from 128 to 256, I have also unintentionally doubled the 'sea level' (I do not mean the actual height of water, as I am well aware of how to change that, but rather the height of typically flat terrain) which is normally around 64 but has now increased to around 128. However I do not want the majority of terrain to be raised, only mountains and hills, meaning 'sea level' needs to return to the original 64-ish blocks.

Here is an image to hopefully better explain what I'm saying.

Does anyone know how I might fix this? I want to keep the 256-high terrain gen but also to lower the sea level back down to its original height. I suspect it's as simple as changing a single number in the noise field generation, but I have failed to find a solution after working on it for around three days.

Here is the ChunkProvider code: http://pastebin.com/PSubPwWu