I have an SE/30 that can boot into 6.0.8, 7.1, 7.5.5, 7.6.1 and 8.1 via System Picker (I can boot 7.6 and 8.1 because I have a IIfx ROM). I can tell you from years of hands-on experience that dealing with 400k floppies in any flavor of System 7 is problematic at best. I always boot into System 6 to get the job done. You may be able to read 400k floppies on System 7, but to make reliable 400k disks, I strongly recommend System 6.
Buy a cheap Mac Plus with a System 6 floppy on EBAY or Craig's list if you want to make a lot of 400k floppies. I'm sure you'll find a separate use for the Mac Plus as well.
But if you don't have the space (or if your wife doesn't like the idea of yet another vintage Mac around the house), you may find this thread of interest. I've never tried it myself, but it looks like a good solution for those who've tried it:
viewtopic.php?p=145926
Buy a cheap Mac Plus with a System 6 floppy on EBAY or Craig's list if you want to make a lot of 400k floppies. I'm sure you'll find a separate use for the Mac Plus as well.
But if you don't have the space (or if your wife doesn't like the idea of yet another vintage Mac around the house), you may find this thread of interest. I've never tried it myself, but it looks like a good solution for those who've tried it:
viewtopic.php?p=145926