To shorten a Rc31 by 20mm is easier than you think, if you can't or don't want to do it yourself Tim at Fork English will gladly do it for you & your 8yr quest will be over.
C-Type 420mm is very interesting ;-)Will have some 420mm but not C-Type for sale over winter. Latest Pace iteration.
However, while it's not for me to say really, the thread serves. Pretty sure there is one set of C-type 420mm left on the planet and have a rough idea where they are now.