If you look behind the fuel pump (Pontiac motor) and under #1 cylinder exhaust port, it is stamped from the factory in large type letters "400", "455", etc.... on the side of the block. If you reach under there with your hand and press your fingers real hard, it will leave an imprint of the numbers on your fingers and then you can tell which motor is in the car. I know it sounds goofy, but it works and I do it all the time when I find a Pontiac out in the fields and can't crawl under them to look.