I should have thought of this the first time I saw the thread. I don't have a clue who to ask for wartime Mulliner RAF bus drawings, but I know someone who does ...
The VK62B Dodge belonging to Rick Jones had a wartime Mulliner body, and as you can see here;
http://www.oldclassiccar.co.uk/dodgehistory.htm
... he got a copy of the body drawings from somewhere. Find Ricks e-mail - he is the admin of that site, and ask him where he got the body drawings maybe?