I have seen body shop workers take car hoods, doors, fenders and other wrecked parts that had curves and or bends in them and cut the roof out and weld the scrap pieces in to look flawless. I was amazed.
My sport hood on my m46 car I just painted had a rectangle cut out of the middle between the scoops , all the way down. It had a blower sticking through it. The body shop guy took and cut the center out of a 68 coronet hood and welded it in. You cant even tell. Same line and all. So the best thing to me is cut the rot out so it is gone and replace with real metal