The last Macs that could use any auto-inject drive were the Nubus PowerMacs. I had a 7100/80 with the shell of a Centris 650 and an auto-inject Sony floppy drive inside. Worked fine. Now, whether or not the 400K drive is compatible, good question. The 800K drives probably would work, but you'd have to run 7.5.x or older to initialize disks as single-sided, IIRC.

-J