The answer has always been somewhat vague. On the ends.

Hopefully this will help. This is a cutaway of a Chrysler truck head. It is actually casting number 1733463, and typical of this type of head. It has always been thought that you would need to drill close to the extended casting in the corner to hit water.
As you can see, the entire end is open for water. Hope this helps.

Another thing that may be of interest is the oil drainback hole. It can be enlarged, but not by much.