I was looking for one like that when I was putting together the FIA race car and could not find one. After talking it over with some people who were knowledgable concerning the bar and its placement in order to provide proper support in case of an accident and especially rollover, we welded a bar between the legs of the roll hoop below the body as you want downward pressure on the shoulders. You would not need the seat back brace welded to the bar. I needed it to bolt the race seat to for support.
Being that this is a lot of work, and you are ending up with the shoulder harness basically in the same place as going to the rear bulkhead, I would suggest the latter.