How can I get more HP and torque on a 2005 Chevy Silverado z71 crew cab

Cold air induction. Exhaust system. Get the ECU reprogrammed. Start there.

Better breathing is a must. Larger and less restricted intake, and larger and less restricted exhaust. Check the on line test results. 15 to 25 HP gain and torque along with mileage. There are low restrictive cats that also can be used. If you really feel the need for speed, look at turbo charging. There are ones that hook up in the rear of the vehicle and the return lines cool the air so less problem of hot air reaching the intake. For all that money, ten grand will get you a huge stump-pulling crate motor. Start with the exhaust and intake air flow and a reprogrammed chip.