Sorry, I can't really help, but it looks good, and sounds fun. make sure you add a fire effect in his mouth. For shots for basing the modelling off of, you could probably just guess and sketch up something, or copy it roughly from the movie. I don't think tolkien ever drew anything concrete, but the description should suffice.
Only problem is, Grond was mega huge, and why? Because the gates of Minis Tirith were...mega huge. My epic wall gates defending Rome are about...well not mega huge...It would be really sad to see a scaled down souvenier version of Grond being powered by 8 little pasty guys.