So which year is the silverado you bought this engine from ?

According to wikipedia GM's 5.3L build back in 1999 was rated @ 270HP and 315lb of torque.

Then in 2000 GM upgrade the engine to make it kick out 285hp and 325lb of torque 'till 2003.

In 2004 GM make it up to 295hp and 335lb of torque.

I guess the one you gonna use is from a '99 silverado isn't ?