* Ford 351 Windsor: The cylinders are numbered 1-8 starting at the front passenger side (right side in a RHD vehicle) and going sequentially to the rear passenger side.
* Ford 351 Cleveland: The cylinders are also numbered 1-8 from the front passenger side (right side in a RHD vehicle), but they are numbered in a different order than the Windsor. The Cleveland's firing order is different which leads to the different numbering appearing on the valve covers and other components.
To be certain, you need to identify *which* 351 engine (Windsor or Cleveland) you have. There are visual differences, but a casting number is the most reliable way to be absolutely sure.