I have 2 indexable internal threading bars. I don't use HSS due to a preference to have no grinding indoors (tiny old house).
First is a 3/8" x 4" LH threading bar (Shars SKU: 404-2715) using 11ER inserts. I use this 90% of the time for cutting right hand internal threads away from the chuck. I leave the threading direction the same as cutting normal right hand threads, but I run the spindle in reverse and cut on the back wall of the bore (opposite the operator). The tool is facing upwards like normal.
This might seem complicated, but try it once and you'll do it for all internal threads.
I also have a 1/2" x 7" RH threading bar (Shars SKU: 404-2290) using 16IR inserts. I use this for larger bores and through hole internal threading of right hand internal threads towards the chuck. I bought this before I knew about the threading away from the chuck trick, so I don't use it quite as much. I may pick up the same tool in the LH flavor at some point.
Mike