How much new hardware will you be putting in the Classic!?
You can't seriously expect to run any sort of Ubuntu on an m68k, and by ripping out the CPU you defeat the purpose of having the Classic.
You can still after all get a Linux distro (really old version of Debian) for the m68k that will probably run on the Classic.
As for the G3 Macs, just put DebianPPC on them or something, Ubuntu is too much.