Hi JFB -
(Not having read your list - yet...)
My suggestions might be:
1. Reinforce all Jack Points, especially the one at the back of the car (the box beam that terminates the frame). There is a standard size rectangular bar that fits right in there, see one of the early pages of my 5357 thread.
- JFFR reinforced his forward jack points, and tied these into a stiffening system of his own design to provide a stiffer frame, without the wacky stiffening kit that takes up the room in the luggage compartments.
2. Seam weld the car, at least in areas that supply structural stiffness. This can be a big project, but your car is in the state where it can be done.
3. Improve / strengthen suspension mounting points - like "Push1267" did on his Group 4 car over there in Denmark.
4. Check the areas of the car in front of the engine compartment bulkhead on either side. There is a big hole where debris can get in on either side of the engine bay front bulkhead, and the debris lays right agains a single thickness body panel.
5. Drop the battery below the front trunk onto the steering rack - you fabricate a tray and a door for it.