XPost Facto?
I did that about ten years ago with my 6500. It's running 9.2.1.
OS 9 Helper. Xpostfacto is for installing OS X(up through Tiger) on unsupported Macs, but there's no chance of getting any retail releases of OS X to run on a 604e. I did briefly have Rhapsody 1.2v3 installed on my 8600, and may still put it on either this or the 8600(although I really want to copy Intell and put Leopard on the 8600 with a G4 upgrade I have on the way).
The problem I kept running into was that I installed OS 9 and 9.1 using North American disks. OS 9 helper requires the 9.2.1 and 9.2.2 update be .smi files on the desktop of the target computer. I have these on disk, but couldn't locate the .smi files from where I downloaded them a long time ago. I tried to get them off Apple's site, but although I found the pages, the download links were dead.
I went trolling on the internet and found a mirror of the updaters(they were released for free), but the site was based in the UK and had the international updaters and not the North American ones. These would not install over the North American version of OS 9.
I have a retail OS 9 International install disk, so I formatted and started fresh with an OS 9 install. Unfortunately, I only have a North American 9.1 disk, so was stuck there as it wouldn't install over the international 9.0 install.
Right around that time, member MacTech68(who is in Australia) sent me a link to his personal FTP server with the North American 9.2.1 and 9.2.2 .smi files on it. Thanks to him, I was able to download those files. I then-again-installed 9.0 NA, 9.1 NA, and finally was able to use OS 9 Helper to install 9.2.1 and 9.2.2. It's actually a pretty significant improvement in performance. The OS itself doesn't see any improvement from having two processors, but the dual processors perked up Photoshop 6 really nicely.