Thanks for all the info. Just to complete the story I now have a 1976 el camino with 1968 Olds 455.
BTW I found a very informative site, that has quick visual rules of thumb for identifying different GM engines without recourse to casting numbers etc.
See the link below, page down about 5 times and click on 'similarities:other GM engines'. Perhaps it of use to to others and could be put in the knowledge base?
Courtesy of 442.com