Installed the 24" titan thumb.
I was concerned about stressing the dipper stick since they don't sell a thumb for this model but as it turns out, the mounting plate pretty much spans the length of the dipper stick so there is no real concern of bending.
I did find that the original pin locations didn't appear to allow the thumb to extend far enough in my opinion so I added another set on the brace mounting and the thumb pivot point. Installed it yesterday and quite happy with the results, and surprised how much it can lift.... If the bucket and thumb can get around it, it will hold and lift it without an issue. I can see that it would be easy to tip the tractor if you made sudden movements holding a large rock, with the boom at maximum swing but there are lots of ways to tip a tractor.
Pretty happy over all, I may notch the bottom of the thumb for the centre tooth at some point so it will close against the shell of the bucket but so far it seems to do quite well. I may try making one for the JD400, it would be nice to be able to lift 1000lb rocks and have more reach but for $150 I am very happy with this.