[JRSully]

They use a special "Bridgeport" machine. Basically what is done is they us the standard decking/milling machine that does the circular motion. Once they get close to the end (stamp pad area), they stop that machine, draw a line where the head ends and the stamp pad begins. They mill the remainder of deck with that special Bridgeport machine that mills/decks in a straight line and they bring it right up to the line they scribed, leaving the original stamping/broaching intact.
make sure you have a guy that has done this before.! This what they did on my 70 L78. Good luck SULLY