I used to stamp a lot or replacement small component data plates and I always marked apiece of tape and laid it across the boss in a straight line before stamping characters. Even then it can be tedious & difficult. I'm sure the Ford guys that did that had their good days and bad. It's not as easy as it may seem to get a straight line. A person has to be well acquainted with the stamps, have a good eye, and be very proficient with a hammer. The hammer thing can be the most difficult part when doing it in situ. It depends on how the engine was secured. Ford generally stamped them after they passed the rotating resistance test at the end of the engine line.