Agreed, non trivial!!! I'll see if I can do something...
Edit: Here is the subpart with regenetated top surface matching studs. I let you do the rest...
But there's a lot of problems with this part, such as studs not on grid (maybe they are not in real life? I don't have the part...), and bad middle split (you should convert middle triangles to quads before splitting part!!!).
Otherwise, I used this method to create filling between head and 4-4ndis.
First color ndis and edge of head hole in different color (eg. red and yellow). Then I added triangles one at a time in MLCad, visually adjusting them approximately (using a 0.1ldu grid). This is pretty fast: once you have a triangle (again in a different color, say cyan), duplicate it, and pull one triangle vertex close to the next structure vertex. Don't forget to invert BFC each time, the operation of pulling only one vertex inverts winding! When you're done, put all newly created triangles to the end of file (in MLCad, select one cyan triangle, then select same color, then cut (ctrl-x) and paste (ctrl-v). Then reopen file with LDDP. There, select everything but the newly created triangles, then Tools -> subfile selection. Name your file, eg. snap.dat. This creates a subfile that will be used to snap triangle vertices to the structure, using Unificator. Don't forget to save main file (which contains the triangles and one reference to snap.dat), as LDDP doesn't do it automatically. Then open main file in LETGUI, tool Unificator, option "snap on subfiles", subfile unification threshold = 2.5ldu (may need adjustment). After that, in the result, inline snap.dat. It then remains to re-generate condlines using Edger2...
Edit: Here is the subpart with regenetated top surface matching studs. I let you do the rest...
But there's a lot of problems with this part, such as studs not on grid (maybe they are not in real life? I don't have the part...), and bad middle split (you should convert middle triangles to quads before splitting part!!!).
Otherwise, I used this method to create filling between head and 4-4ndis.
First color ndis and edge of head hole in different color (eg. red and yellow). Then I added triangles one at a time in MLCad, visually adjusting them approximately (using a 0.1ldu grid). This is pretty fast: once you have a triangle (again in a different color, say cyan), duplicate it, and pull one triangle vertex close to the next structure vertex. Don't forget to invert BFC each time, the operation of pulling only one vertex inverts winding! When you're done, put all newly created triangles to the end of file (in MLCad, select one cyan triangle, then select same color, then cut (ctrl-x) and paste (ctrl-v). Then reopen file with LDDP. There, select everything but the newly created triangles, then Tools -> subfile selection. Name your file, eg. snap.dat. This creates a subfile that will be used to snap triangle vertices to the structure, using Unificator. Don't forget to save main file (which contains the triangles and one reference to snap.dat), as LDDP doesn't do it automatically. Then open main file in LETGUI, tool Unificator, option "snap on subfiles", subfile unification threshold = 2.5ldu (may need adjustment). After that, in the result, inline snap.dat. It then remains to re-generate condlines using Edger2...