The lower brace can go on after you get the basic skeleton put together and are ready to fit the skins. At that point, you will want to get the sag out of the center of the wing, and get any twist out before you start match drilling. Just run a plumb bob down each side of the wing from the main spar, measure and make sure each side is equal after you have leveled everything. Finally, clamp the rear spar down to the lower brace to keep it there while you work. Check it again before you rivet because it will probably move a little with the jig being made of wood.