Neil, what I see there makes me very worried about the overall integrity of that frame, not to mention the sheet metal parts. Is a frame-off possible for you? If you remove the trunk floor, I'll bet you will find substantial rust at the rear crossmember, to which the rear springs mount.
Additionally, you should give real thought to opening up that frame to get some rust treatment, so that all your hard work will not just be a temporary fix. Rust never stops.
You have my sympathy and envy. I thoroughly enjoyed the year it took me to fix up my frame, pans, etc.