As an editorial comment, there are too many good cameras in the world to be messin around with... Duaflexes. It was designed to be a crappy camera, well executed as a crappy camera, and it hasn't aged well. I have been tempted though.
When I get the urge to shoot something 'old', I reach for the rb Graflex or my Zeiss Trona, both pre 1930, both capable of producing better work than I can, both giving the film that little bit of age induce character.
To answer your question, I would shoot the 400 and stand develop in Rodinal 1+200 for several hours. That would be a unique look.
tim in san jose