Capping the base damage makes sense. I'm still leery of allowing something that heavy to cause solely non-lethal damage. It doesn't fit the rules well. Falling damage is dependent on how far you fall. Most creatures can ignore the first 10 feet, but this ignores the first 40 feet. That's already a bonus. I'd say let the falling damage be 4d6 (and it ignores the first 40 feet so unless it falls off a very high cliff it should be okay)
It would make sense to be all non-lethal if it were spongy (and weighed a lot less).
This could also use better description.