I had the same problem on my -70.
Ended up detaching the trunk lock bracket and the license plate "box".
Made a patch panel and welded in.
Sandblasted the license plate box, painted the hidden areas and welded it in.
There was rust under the tail lights, around many of the holes for the trim.
Patched all these areas.
Some leadwork and shaped the edges.
A lot of work but it looks OK.