Sorry I am a bit late in replying, I've just recently registered. I bought an '07 Silverado Classic LT w/ the 5.3, but prior to that I had an '01 Pontiac Trans Am w/ the LS1 5.7 engine. As far as I am aware (someone correct me if I'm wrong) the 5.3 is pretty much a smaller version of the LS1. I sold the car with 167,000 miles and the ONLY problems I ever had with it outside of routine maintenance (bought it brand new) was the cat./convt. and a fog light. The engine felt just as solid as it did the day I drove it off the lot and the car could still cover the quarter mile in under 14 seconds. To be honest, it was the most dependable car I have ever owned hands down. It did have the piston slap, but even that didn't stop it! I wanted a truck and the Silverado was a no brainer based on the fact that it had a 'baby' LS1 in it and the dealerships were significantly discounting them. I plan to put about the same amount of mileage on the truck as I did the TA and I do expect to be at least as dependable. These LS engines can take a lickin and keep on tickin. The transmission may be a slightly different story, as I'm finding that their life expectancy is somewhere between 100-150k miles. Keep your fluid changed regularly. Another common problem seems to be the rear pinion seals and u-joints. I did have a slight pinion seal leak on the TA, but the driveshaft, u-joints, etc. were fine... so I'm not sure why the trucks are any different.
Also, even though the manual says it is okay to use 87 octane... I find that the LS engines don't like anything below 93. My Trans Am would ping up a storm if I even used mid-grade. I started off using 87 in the truck and now even it is starting to ping! (Argh!)
In any case, you should be fine. Enjoy your new truck! It should last you 200,000 miles+ even if you don't take care of it! The LS1 in my TA saw redline multiple times almost every single day and ran on regular dino oil which got changed only when I got 'round to it.