Try here:
https://books4cars.com/listings.php?findmake=Mercedes

The W116 body manual does have body/unibody dimensions.

It doesnt have anything about replacing sheet metal panels.

search this site and others for restoration blogs.

https://www.benzworld.org/threads/1973-w114-280-rust-repair-journey-with-pictures.3098745/page-2#post-18453401

I have some pictures on my 6.9 resto blog of rocker repairs. I'm just finishing up the passenger side now.
https://forum.w116.org/test-drive/my-6-9-s/msg122321/#msg122321