Mac OS 9.1
Before you can upgrade to Mac OS 9.2.2, you need to install Mac OS 9.1. I recommend this because I recommend Mac OS 9.2.2, and I recommend Mac OS 9.2.2 because it is much stabler and faster than any previous version of OS 9, and allows us to do more upgrades and enhancements than any version of Mac OS 8 or System 7 does.
My 3400c runs Mac OS 9.2.2. Before you attempt to install Mac OS 9.2.2, though, there are some issues you should read about in the Mac OS 9.2.2 section of this website.