I just finished doing metal work on the right and left
lower window patches. I used the large , lower corner patches you pictured. The work came out great. The mouldings even fit
the round corners very well. This was on my 68 Coronet.
If only the front part of your dutchman is rotted use the front
piece that is pictured or if everything is rotted get the roof and all the other pieces you have pictured.
Here is a link to a thread on this kind of repair on "For Bbodies only".
Look for the postings by GMP440; that's me. It shows pics of the lower corner repairs.

http://www.forbbodiesonly.com/moparforum/threads/69-rr-b-body-rear-window-corner-repair.119437/