Happy Birthday Leeler.
I doubt that a free showing of that movie will bring anybody to the theater.
What I would do: Play for free, or $.70, whatever the price might of been then the latest big movie. Play the old movie pre-show for a bit for a few minutes of neatness. Maybe for the week, especially if you can arrange it to when it's one of your slower weeks.
Dress up in 1940's clothing. Have 1940's gadgets around. Play a slideshow of how much things cost in 1941, etc.
Certainly a very cool thing to do.