Help splitting single 2-item mesh into 2 meshes

[Vyxenne]

I have a mod-added two-piece (top and pants) outfit that I converted to SSE from Oldrim by using Ousnius's Nif Optimizer and resaving the plugin in the CK64. It works fine in game, but it was made for a totally different body than mine. To fit it to my body in Outfit Studio, I have to move the top UP and the pants DOWN o_O but I can't do that because the existing mesh only has one shape which includes both the top and pants.

 

So I imported the converted nif into 3ds Max, deleted the pants and then exported what was left (the top and body) as its own nif named 'Top_1.nif,' but when I opened it in NifSkope to correct the biped slot I found that the mesh had somehow lost its BSDismemberSkinInstance block. So I opened the original mesh (pants and top combined) and hit Block > Copy Branch on its BSDismemberSkinInstance then selected the 'Top' BSTriShape and hit Block > Paste Branch- but instead of 'landing' correctly as a child of the 'Top' shape it landed as a top-level node and I can't find any way to 'tell it' that it needs to be a child of BSTriShape 'Top.' I can't find an answer to this in NifTools tutorials, and of course my top is invisible in-game because it has an unspecified body slot. So I have two questions:

 

How can I import and export the mesh in 3ds Max without losing the BSDismemberSkinInstance block in the process? Why is that Block disappearing in the first place? It isn't visible to me in 3ds Max so I can't see any way to deal with it- how can I re-create (or resurrect) it in 3ds Max?

 

====OR====

 

Once I have my new exported mesh with only the body and top in it (having deleted the pants in 3ds Max), how can I either create a new BSDismemberSkinInstance for the Top shape in NifSkope or copy > paste one from a similar mesh and have it be a child of the shape instead of a top-level node?