I'd probably leave the SE as is. If necessary, either use a ZFP case for a hard drive, or install an internal. The 800K drive will read/write MFS 400K disks no problem. Set it up with 6.0.8 or a light install of 7.1, and it should work fine.The PowerMac I am trying this all out in is my 5215CD. 7.6.1 installed, will probably move down to 7.5.1 (the oldest it supports). This Mac has a an archive of all older systems and utilities for creating 400k disks on a second internal hard drive. I think I will try 800k drives and see what happens. I did try the 400k anyway - and everyone was right, no go. Powers up but does not spin up.
But I have an SE coming (dual 800k), so I may delegate that machine my new work horse for vintage floppy production. I have a spare 400k. For simple use, I may instal that drive in there and install an internal HD. Then I will see if it will run and recognize it as a 400k. I know its not needed to make 400k, just me being weird.
Re: the 5215CD, I suspect it has the same controller chip as a 7200 or 7500, so it probably will just continually run the eject motor on the auto-inject Sony mechanisms.
-J