I had the same problem on my '66, but not to that extent.
I could not find the part as a new replacement in '97, so I fabricated and welded sheet metal in myself. The hardest part was getting all the dead metal out and blasting the cancer away.
A friend of mine was able to get this section for his '65 from a junk car, even here is FL. This part included the body and structural sections between the trunk lid and top. Just had to cut the old out and weld in the replacement.
Good luck.