According to earlymacintosh.org, Mac OS 7.6 dropped support for writing MFS format. But from what I'm reading on other guides it sounds like you should be able to make the image... Do you have any 800k disks that you could give a try? As the article states, you can copy 400k images onto 800k disks.
Ah here's something: http://68kmla.org/forums//viewtopic.php?f=7&t=18023#p169328. Perhaps the disk images you have weren't made with DiskCopy. If that's the case, it might be worth giving DiskDup+ or ShrinkWrap a try.
See if you can mount the images (make sure the files are locked to preserve the images,) on the G3 clone using DiskCopy. If not, then you know the images were made with something else.
Ah here's something: http://68kmla.org/forums//viewtopic.php?f=7&t=18023#p169328. Perhaps the disk images you have weren't made with DiskCopy. If that's the case, it might be worth giving DiskDup+ or ShrinkWrap a try.
See if you can mount the images (make sure the files are locked to preserve the images,) on the G3 clone using DiskCopy. If not, then you know the images were made with something else.