I think Soap's asking more about how to set up his box to work as a gateway or a router for his network, not spoofing. You're best bet would be to check out the Linux Router Project, found at http://www.linuxrouter.org
Unfortunately the server is down at the moment, so I can't get the exact link for setting up your box as a router/gateway. I also found a pretty good page with instructions on setting it up, it can be found at http://www.escalix.com/freepage/doggerdog/freesco/
The guy uses the Freesco distro, but you can use whatever unix you want.