Before you make any changes to this file make sure System Preferences is closed and Internet Sharing is disabled.
Here's an example of the final plist file in /Library/Preferences/SystemConfiguration/com.apple.nat.plist.
You have to add the new values to the Airport group. After I enabled Internet Sharing and connected the IP I had was in the correct range.
NAT AirPort 40BitEncrypt 1 Channel 0 Enabled 0 NetworkName NetworkPassword SharingNetworkNumberStart 10.0.5.1 Enabled 1 NatPortMapDisabled