This is a '76 but is it just that part down at the bottom, in the middle?
If it is, you could do option #1 fairly easy but with 18 gauge sheetmetal or some metal off another car. Something of a similar thickness. I wouldn't get a new part to cut up just for a 6" hole. If it has some funny contours, post a pic. Something under the car like that would not have to be just perfect.