IP Spoofing

Been trying to change the TCP Header Source IP-Address field in my packets for couple days now. But it seems to be quite hard. I need to do a HTTP Request with Spoofed Source IP-Address. If possible could it be done for Curl?

It isn't that simple (no matter if with or without Curl). If the server you are targeting is using the right OS it is basically impossible.
http://www.phrack.org/issues.html?issue=64&id=15#article