If you could see inside the cases, you would see the other area where they bore through the cases, just like either side. This area is closer to the left side, so, if you go the two-piece route that is where the two pieces butt together and are supported.
It can be difficult sometimes to bore bar the custom ID of a full length tube so usually you go the 2-piece approach.
I'd imagine its possible to CNC a full length tube using an indexing boring bar maybe.
The dimensions I gave you are spot on.