The original engine for this truck would be a 216, the 235 engine available during that period was 1-1/2 ton or bigger (Loadmaster) only. Serial number and casting marks in the block and head will be the way to tell.
In this picture you can clearly see the number on the pad as described.
http://www.flickr.com/photos/advance-design_parts_co/8473963443/in/set-72157632770756320