Bostonian 71 demon

What is the back of the new frame rail welded to? Previously there was nothing there, no? Looks like you extended a plate inward to accommodate the new end point?
So the rear section of the new frame rail is welded to 6 inches of the old rail parallel to each other with a seam on the top, rosette an front and back welds on the inside, the angled portion comes and meets the end of the new rail at a notched portion so that way it's not just a smooth cut off the top is also plated an Rosette welded together