Issues
Mac OS 9.2.2 Does not Install on Pre-G3 Machines. See Mac OS 9.2.2 section for fix.
Also, Mac OS 9.2 and above disables sound on 3400c. There is a downloadable fix, thankfully. See Mac OS 9.2.2 section for details.
Computer will not boot or reboot with a 32bit CardBus card inserted. It also seems that 32bit CardBus cards can only be inserted in the lower slot, and that the 3400c becomes unstable if trying to shut down or reboot after using the FireWire card.
Do not try to force insert an unmodified 32bit CardBus card into any of the slots without shaving plastic off of them first (see FireWire section). Even then, I would only suggest you insert them into the lower slot.