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It is a 5.3, and you need to realize these numbers are at the wheels. The stock heads are fine, they have the best cfm flow to combustion chamber size for a 5.3. Aftermarket heads tend to make more horsepower at the expense of low-end torque....which is fine in a lighter car. In a truck/suv you want all the torque you can get. The 706/862 is the perfect head for a sub-500 hp engine. A 400 hp AT THE WHEELS 5.3 is closer to 500 at the crank. And that my friend is a rowdy engine.The fella in that video needs to add some aftermarket, better flowing heads to the headers and e50 fuel to get to the 400 hp mark. Not sure what motor he's tuning either.