Can I use InstaDMG to create a 10.5.8 image on a 10.6.2 host

Hi, I have a need to downgrade my Mac Pro (Nehelam Xeon) which came pre-installed with 10.6.2 with 10.5. I have a 10.5.2 install disc but I figure I might need 10.5.8 to get any updated drivers etc. I have never used InstaDMG before but would it be possible to create a 10.5.8 image (I converted the 10.5.2 install DVD to a disk image via disk utility and downloaded the 10.5.8 combo update). I then plan to copy this image to a firewire disk and then boot the Mac Pro in Target disk mode Thanks in advance