If I'm right on the sizes, this conversion would result in a model that has arms that remain roughly the same thickness the whole way down. However, some sculpting of the elbow will be needed to blend them together and get the arm length to look right. The barnacles by the elbow may help with this, too, I'm not sure.
Mollog's staff will also need to be cut and repositioned in the hand a different way so that it looks correct (most likely so it is pointing downwards and touching the ground). With the way I think this would be assembled, that would have the staff coming out of his hand by his pinky, like you would normally hold a walking stick. Also, the blue and white mushrooms from the top of Mollog should be swapped in for the two large mushrooms on top of the Dankhold Troggoth (and will need some sculpting to repair and blend in, since they're currently attached to one another).
Finally, the model should be mounted on a 50mm base, rather than 60mm. This is because I'll be using it as Mollog and that's the largest base size that fits in a hex for Warhammer Underworlds! Ideally, this would just use Mollog's own base (perhaps cut into a bit so it doesn't look like the model is just perched on top) but could use a blank 50mm and have detail sculpted to match Mollog's base style, if that is better.
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So, as you can see this basically requires a sculptor to blend organic parts together, and make a model look like it was created to be a way it was not

. Sculpting at the elbows, to get the arm length looking right and blending the parts together, will really make or break this conversion!
These are both "Dankhold Troggoths", I just really want to use the big one

and think with the smaller arms/hands it should both make it look better as a model (in my opinion) and be better for gameplay in Warhammer Underworlds, since it needs to feel right on a smaller base.
I'm looking for someone with availability soon (ideally early February!) for when both models will be released. Ideas, referrals, etc are all welcome! I would hope to see some examples of sculpting or serious organic model conversions from the person, to know they could handle the job.