As far as I know there is no 6.0 in the 04 GTOs only 5.7L. It wasn't until 05 or 06 that they started the LS2 in the GTO.
Other than that just look around this first page and you will find two or three informational topics that will give you A LOT of information on what is needed.
You will also find a few swap threads that are very informative.