floor pan firewall question

Cut where the metal is usable just above the rust Make sure to give your self enough room to flange the doner metal. I assume your welding it your self. Harbor Freight make a nice air flanger. Flange the doner metal then drill or punch hole in doner flange to weld new metal to old. It would be hard to butt weld this as the floor is very thin. Also use weld through primer before installing and welding to slow down new rust. East wood and most body supply shops sell weld through primer in spray cans. Do as recommended above seal it well both sides after installation.