Devin
There is a difference between the "casting number" and the "engine code" which is what I think you are looking for. If you are looking for the engine code, which will tell you what kind of car or truck the engine originally was installed in, then look at this site for some pictures:
http://www.nastyz28.com/chevy-engine-code-stampings.php
If the area where the code should be is blank, there is always the possibility that the engine has been remanufactured and the top suface of the block was remachined to make it flat. This will cut away the numbers.
If you are looking for the "casting" number, it is typically on the flat surface right behind the drivers side cylinder head. Some are also on the passenger side. You should be able to see it from above by looking down at the back of the engine right in front of where the transmission bolts up. This number really does not tell you much as the same casting number block was sometimes used for many years.
Hope this helps,