You will need Carbide Create Pro:
then you will need to model the stock:
https://carbide3d.com/hub/courses/create-pro/3d-tool-add-shape/
then it should work to subtract the STL at a suitable position:
https://carbide3d.com/hub/courses/create-pro/3d-tool-import-stl/
trace the outline at the desired level — this is a new v8 feature:
and use that outline to set the balance of the shape as desired.
If you get stuck, upload the files and we’ll walk through this with you.