Here is a web site that I've found useful in sorting out Chevy 6's these engines because of their long life and interchangeability are often a mix of years before you even start working on them.
http://aera.org/techtools.aspx is a site for checking casting numbers there are also some other good casting number sites. One of the first things I do is check all the major casting numbers to see if the engine is a unit of all the parts atleast coming from the same year.