[19Fordy]

I would think that the best way would be to bolt the good top pieces in place on the car
and then cut and tack weld the replacement bottom pieces (in position)and weld in place as much as possible. Then remove the entire assembly and finish weld on both sides. Lap welds collect moisture.

It would nice if you cut and weld the seam right behind the stainless vent trim pieces so that it would be hidden.