The harness should still be similar, differences would be where the connections are for the alternator, the connections for the trans if the engine came with a 4L80E, the injectors use a different connector but should be in the same place. And they didn't go to the DOD in 04' so that makes it a bit easier. What exactly are you trying to figure out? Let me look through some of my old pics, I used a 5.3 for a build in my 69' Pontiac a long time ago, maybe I have some pics of the wiring from that.
Well, the injector connectors I am changing anyways as well as the alternator, so those are easy. The issues I am having is just where I am trying to label the wiring harness connectors based on that picture I posted, and the connectors aren't there, or there are connectors that are on my harness that aren't in the picture. It would be easier if I had the other ends of the connectors to match up against, but I don't. Thee are also a fair amount that I have to convert since the oil pan and intake manifold are different. So is the MAF and so is the throttle body. Fortunately I just bought the last "missing parts" I was waiting on to actually complete the engine itself-the MAP sensor and the EVAP system. These have to be present since the DMV requires all factory sensors and emission systems to be in place.