My personal preference would be to cut the new panel to match what you have left. Leave as much Ford panel as possible. Secondly, I prefer to do an overlapped joint along that long horizontal cut. I do not have enough confidence in my grinding skills to stop grinding a butt welded joint before the surrounding metal is significantly weakened. I have read that people use the butt weld joint because they are concerned about the overlap not being original. But if you look anywhere on the car, Ford overlapped adjoining panels (obviously not in this specific area). The structure was compromised the minute it was allowed to rust through. Cut it, Fit it, Weld it, Seal it, Enjoy it.