2001 to 2007 Engine options
 

I am looking to buy a used GMC 2500hd from 2001 to 2007. One of the reasons for the 2500 is for the 6.0L motor as opposed the the common 5.3L in the 1500. I realize there are some 1500s out there with the 6.0L in it but they seem very few and far between. While doing some of my looking I noticed for example that a 2007 1500hd had a 6.0L that had 300hp where another 2007 1500slt had a 6.0L that had 367hp. Can enyone give me an idea of how I can tell when looking at either a 1500 or 2500 from 2001 to 2007 with a 6.0L motor, what hp rating it will have. Which engines came in what trucks, etc.
Thanks in advance for any help.