From what I can remember, at that point in the car, you have the Z section chassis with a 1" by 6" plank of ash bolted on top of it followed by the Ash frame at the edge, which supports/shapes the body panels. (you can just about see them all in the picture below, on the pasenger side, the back is chassis, the blue on top is the ash plank, just at the edge you can see a slightly different color about 1" which is the inner body frame.)
It's hard to see from the picture, but it looks like the crack has started at the top.. as this is where the gap looks largest. If this is the case, then it looks like a stress fracture in the body-pannel (which I'm assuming is Ali) where the tail end of the front tub pannel goes under the door.
