The terrafirma big bore shocks have a much larger bush than the land rover standard shocks. You're right though, the upper bracket pin is the same.
The washer holding them in is smaller than the bush and has already once popped out on us - terrible design!
After trawling lr forums, it seems others have experienced the same problem so I'm looking for a way to solve the problem if it happens again... I'm hoping for a much cheaper solution to replacing the shocks!