Macs are designed so that you cannot "downgrade" the user experience by using a version of the OS that shipped prior to the introduction of the machine. You can probably go into the files on the disc and look where the allowed machines are listed and alter that file to include your target machine but I can't guarantee that will work. Some owners of 601 upgraded Quadras have gotten 8.5/8.6 to run that way even though Quadras aren't included as supported machines. It's also been done with OS X installs to get Tiger to install on Macs without Firewire. I am not sure if that works with downgrading the OS, though, as the examples above are clearly upgrading the OS beyond what Apple intended not down.