Depending on if you have a truck already and the local market for used SSr's, I'd consider carefully the economics and performance of selling the 03/04 and simply getting an 05 if you have the first generation truck.
There are real questions about the M30 transmission's longevity with the additional power which, I am told, is the reason the 05 comes with the uprated M32.... if you want "go" consider a 1SA '05 with the manual transmission and very limited options... you get the upgrades that come with the 05, including the LS2 engine and the other running changes folks on the site have noted.... good warrantee and better handling et cetera.... all for about the same $$$'s out the door....
It's a no-brainer that if you are looking for the 100 or so more horsepower, the best way to go is to trade up to the '05, and get the beefier transmission, too. To me, the way to go is to trade up, then supercharge the LS2! Then, you would have something with get-up-and-go. Kenne Bell and others already have the supercharger designed and built for this engine. It's just a matter of the bracketry and perhaps the computer programming.
Do any of you money bags out there intend to do this?