Yes, they make ISO 400 sheet film.
Sulfur sensitization will get you into the ISO 25 - 100 range, and will give good contrast with many emulsion types. The trick is to get the right level of sulfur, and coat the right level of silver. Among other things, making the right type of emulsion helps to start with and exposing in mid scale avoids any sharp toe that may result.
The whole process can be as easy as dump and stir, once you get the initial conditions ironed out. It would be best to use an emulsion formula supplied in a text book such as "Silver Gelatin" rather than doing it all yourself from scratch. After all, why reinvent the wheel.