Did the OP try a magnet on the BB shell and/or on those "splined rings" assuming they might be separate components to the shell?
If aluminum then maybe not a good candidate for reaming and threading but, given the wall thickness if it's steel then the fact of it being at most 33/8mm and 70m wide seems like perfect for Italian threading. But you'd need to find a shop who can do that.
Cheaper solution might be one of the no-threads-needed BB units like aforementioned VO, YST or Sunlite.
AND you'd want a shop to machine the chamfers on the outbound BB shell edges, and YES do use LocTite Blue if you go with any of these 3 brands.