i sometimes wonder if its necessary to so rigidly require the matching of safari of OSX versions. Out of necessity? or out of something else?
Somebody mentioned you can try to download 10.5.4 combo update package from apple.com and re-apply it on top of what you have now, that should include safari 3.1.2.
(never tried myself tho, still on 10.4.11 and 10.5.1)