With the old doorskin shed, inspect the frame for rust. A wire wheel and a good rust-adhering coating like POR-15 and some paint works well for repairs. Now remove the remaining flange strips. You can break the spot welds with a metal chisel. If you run into a stubborn weld, you can drill it out. Be careful not to distort the perimeter of the doorframe, which will make finishing the doorskin tough.