Universal I too am looking for such information so stay in touch if you find something please ?
I have a book on order called "Target America: Hitler's Plan to Attack the United States" by Duffy, James P. (2004).Greenwood Publishing Group, 114. ISBN 0275966844.
I have seen excerpts from that book interviewing Ju-390 pilot Hans Joachim Pancherz who lives in Barcelona. That book may provide clues.
Horst Zoeller who maintains this website on the Junkers company may help you too: