The 1965 Pontiac GTO's 389 cubic inch engine (which is approximately 6.37 liters, not 6.5 liters) came in several horsepower variations depending on the carburetion and other options. There wasn't a 6.5 liter 389.
The horsepower ratings varied from approximately 325 to 360 hp. You'd need to know the specific engine code (found on the engine itself) to determine the exact horsepower for a particular car.
