45 minutes should be fine.
If you want more time, then move to an earlier BOS-DFW flight, or the later DFW-NRT flight. Alternatively you can route BOS-ORD-NRT.
Thanks for the advice. This is the 1st flight out of Boston (lv 6:10 am). I will look into the later DFW-NRT. I am upgrading a coach seat with miles, and simultaneously getting a Business award for my wife. There aren't many itineraries that have all that available.