I'm getting ready to do a 2003 Vortec 5.3L swap right now, and the easiest way to do it is to buy a complete pullout with everything from the fan back to the transmission, you will need the wiring harness, ECM and TCM, then you can send them off to
http://www.wait4meperformance.com/ they will trim the harness, reprogram the ECM and TCM and ship it all back to you for between $160-320 they will give you an exact price when you tell them what you want.
Once it comes back you put it in the truck and it runs, no muss no fuss.
Here is some information on the swap,
http://www.lt1swap.com/ if you decide to do this have fun with it.
That is the eaiest and cheapest way to do it by far.