True, it's too tall for a perfectly proportional tank to your source material, but the model's gun is far larger proportionally than the real thing, so the height could be overlooked.
I think if you extend the body it will go toward making the proportions even out more. Though not exactly necessary from a practical point of view (GW models are far chunkier than real world counterparts after all.) It still would make the overall proportions more pleasing to the eye.
There are some threads in this forum from a member that did exactly that, extending the bodies of his tanks. Unfortunately I don't remember who, the title or even how long ago it was that I saw them.
You might also find some inspiration over at the Bolter and Chainsword and other forums on this subject, primarily forums focusing on tank heavy armies.