I figured it out. It turns out OS 8 and newer automatically unblesses the system folder on System 6 disks. I found on program called FixSix but I couldn't figure out how to use it. So I booted the plus from my 6.0.3 disk, ejected the 6.0.3 disk and put in the CopyRoms disk. Then I simply opened and closed the system folder on the CopyRoms disk, which blessed the folder. Restarted the Plus with the CopyRoms disk and now it booted from it just fine. I was finally able to get the ROM from my Macintosh Plus.