funkytoad - the reason you can't make a Mac Plus boot disk on a Plus is that PCs can't write to 800KB diskettes. So what you're going to need to do is make a boot disk on a 1.4MB floppy with the PC, then use another one of your Macs to transfer the system software from the 1.4MB floppy to an 800KB one.

Check out the bottom of this page for info regarding making a System 6 boot disk...

http://home.wanadoo.nl/ruud.dingemans/html/system6.html