Since the space between the inner livingroom wall and the outer wall was the stairwell, the bedroom floor ended at the interior livingroom wall and was supported by that wall. Since I cut it out, the upper floor had no support. So I ran a beam the length of the house, resting on the two outside walls and the two interior walls (only one of which is seen in this picture).
The Glencroft had openings for a back edge anyway, so I just enlarged these to fit my 1/2" x 1/2" beam. Of course, now all my beams will have to run crossways in the house instead of from back to front, but that's fine.