Is there a word for the final stone of a foundation? Analogous to the word "cornerstone"?
the cornerstone is technically a stone uniting any two masonry walls at an intersection, therefore all corners have cornerstones.
For use #2: "a stone representing the nominal starting place in the construction of a monumental building, usually carved with the date and laid with appropriate ceremonies." I would think the last foundation stone is your partner word(s).
I have searched for architectural references, but found only cornerstone and foundation stone(s). Apparently the last stone in a foundation does not deserve it's own appellation.
The opposite of foundation stone (which is originally what I thought you were asking) is the capstone: The top stone of a structure or wall. The crowning achievement or final stroke; the culmination or acme.
From Wikipedia: Coping (from cope, Latin capa), consists of the capping or covering of a wall.
I believe in BrE, it is cope stone.