From inside XPostFacto running under OS 9, you select "Boot from this CD and Install on that Hard Drive". Once you say OK, it should create a "Helper" disk on the candidate Hard Drive, write some stuff like a modified BootX and some kexts on it, boot from it, and begin running the Installer. No intervention should be required.
You cannot boot from the CD. (If you could, you would not need XPostFacto!) The "Helper" disk is what you need to boot from.
Darwin, which Mac OS X uses to boot up, does not actually use a loadable SCSI driver to get itself booted up. It relies on a "standard" built-in driver to get going, then may add a .kext for better performance when running Mac OS X.
Your SCSI driver may be inadequate. You may want to consider installing a modest sized IDE drive to get going, then seeing if you can clone it to your SCSI drives.