You might take a look at a Rodenstock 75mm f/4.5 Grandagon. It will give you an extra half stop over most large format lenses. A 21mm lens on 35mm gives you about 80x60 degrees. The 75mm will give you 80x70 degrees. If you want to match vertical angles of view, go with a 90mm, which is about 70x60 degrees.
For this project, with infinity focusing in a very dark situation, shot wide open, you might find a camera with infinity stops helpful. You can probably rent a Linhof and that lens no problem in any big city.