[DougD]

"I've seen several people selling 427 SO. Some are advertised as service blocks other have been listed as NOS. What exactly is a service block?"

A service block is one that was not originally installed in a vehicle on the production line. It is a repair, or service, block sold through a Ford dealer's parts department or similar. If unused, it would be NOS by definition. A NOS part is a FACTORY part that was produced back then, is not in production now, and is in unused condition, presumably good as new. It's possible to have a NOS part that is damaged, corroded, etc., but that should be indicated by the seller, as the assumption is it's good as new. Many people claim NOS on parts that are still available from the manufacturer. To be NOS, it must not be available from the manufacturer's replacement parts program.