I did the G3 swap myself and it only cost me $100.
Just like
RPM427 said, go to your local autoparts store and buy an alt for a 94 or 95 GT. Here are a few "How To" links with pictures...
http://www.racesystems.com/3gproj/index.htm
http://www.westsidemustangs.com/modu...showpage&pid=8