[jocko]

Most of your questions are answered I believe.
Bottom line - a 71 big block truck would have a 400 badge under the marker light and 396 or 402 on the SPID. If 396 on the SPID, it’s probably early in the production year (but still likely a 402 under the hood). Look at the engine pad stamp and run the numbers to determine if it COULD be original to the truck - COULD does not mean IS. Anything beyond stating it is possibly the original engine is speculation unless it’s the original owner stating it. And I don’t often believe them either. Once you confirm it’s possible, you kinda have to just look at it and assess if it’s ever been removed to build confidence in an “it might be original” statement. Although anything could be swapped on or off, you can also date the carb and other parts to add to or detract from confidence level.