The 3400 is Cardbus compliant: it usually requires a one-wire mod to activate it, but someone may have already done that on your machine. In any case, the fact that your adapter reads okay on a 3400 doesn't prove 100% that it's not Cardbus.
If you can find an adapter that you know for absolute certain is 16 bit PCMCIA and not 32 bit Cardbus, I would suggest installing it with a blank CF in the 5300 and formatting / installing the OS on it there.
Your other option - seeing as you're opening the thing up to check out the HD anyway - is to put a 2.5" IDE to CF adapter in and have your CF hard drive substitute internal, leaving your PCMCIA slots for other things.