I agree with Slowprocess. Under normal circumstances you shouldn't need to worry about a replacement engine for a while.
I just had an engine replaced at the dealer. It was a 5.3, and it was initially replaced with a Remanufactured engine. The replacement engine only ran for a few minutes, when one of th etechs at the Chevy Dealer notices a crack in the block at on of the water jackets. I happened to be there when it happened.
I then thold them to order up a NEW replacement engine for the truck, which they did.
The new Vortec engines are more 'fragile" then the old 350's were, but there is no reason for you not to expect to see 250,000 to 300,000 or more miles out of a 5.3 with only minor repairs. Remember your oil changes evey 3000 miles (no matter what oil you use), and flush your coolant system at least every two years. Take care of the maintenance items and the minor repairs right-a-way, and you shouldn't have any problems.
