Byrd's right - electrically they are all ATAPI drives. However, not all drives are physically ATAPI drives. While 3400 drives are electrically IDE, and show up on an IDE bus, they use a proprietary connector which cannot be adapted to a standard notebook ATAPI interface unless you find the very rare MCE expansion bay module.

PowerBook 1400 6x and 8x drives use a notebook IDE hard drive connector. With an L shaped connector a sled from a 6x or 8x drive can be converted into a hard drive expansion bay module. Alternatively, there are some non-Teac, non-Apple drives that used this connector. I have a Hitachi 24x unit from a ThinkPad that uses the same connector, just plugs straight into the interface board. Have fun getting a 3rd party IDE drive to work in OS 9 though.

As Byrd said, PowerBook 1400 12x drives are really your best candidate for upgrades - they use a standard Micro-50 notebook ATAPI connector like 95% of drives out there.

For what its worth, no PowerBook ever used an internal SCSI CD-ROM drive, not only were there no SCSI notebook CD-ROM drives in the first place, but on top of that Apple had phased out PowerBook SCSI drives with the discontinuation of the PowerBook 500 series. Also, here is a page with some interesting info on various PowerBook expansion bay modules.