In Tiger, launchd hadn't yet replaced StartupItems, so that's what you'll be dealing with. On boot, the system runs the script at /System/Library/StartupItems/Apache/Apache, which checks for the presence of the
WEBSERVER=-YES- line in /etc/hostconfig, and runs
apachectl start if it is. What you need to do is have it use your own version of apachectl, which is probably in /Library/Apache2/bin/apachectl (but it might be elsewhere — hopefully you know).
First, ensure Personal Web Sharing is off in System Preferences' Sharing pane.
Then, taking cues from this page:
sudo mv apachectl apachectl-1.3
sudo ln -s /Library/Apache2/bin/apachectl apachectl
Optional: to ensure the Sharing prefpane is pointed to your own installation of Apache, edit the appropriate .conf file, and add/change this line:
Hopefully that'll be enough to get you on your way.