Have looked right now on the hompage of UUX what I read there it doesn´t look to good for the PB1400 but you are right that it may not be worth the trouble with 64 Meg, though I´ve read that at least the first version of X was installed on an IBook with 64 Meg.
About the 64Meg in my 1400 I have 24 & 32 in the "Upgrade Slot" (the right one) and the original 8 that are soldered on the motherboard. BTW I was able to boot the 1400 once with the original 8Meg from Apple in the left slot which gave me even 72 Meg, was with a minimal install of OS 8.