As mentioned the Harry Weslake designed D5 head require the special D5 block (and pistons) to work. See attached pictures. Block casting no is 3577430 so start looking. They are often confused with the iron "fuel" block so try both search words when looking on ebay, I have seen several there.
Some links with more pics:
https://board.moparts.org/ubbthreads/ubbthreads.php/topics/1268982/1.html
https://books.google.se/books?id=I3WoCgA...p;q&f=false