Here’s are a couple of past discussions regarding steam holes. Additional threads can be found using various search terms in the search bar. You’ll find arguments both for and against the additional ‘coolant’ holes in the block and heads.
http://forums.y-blocksforever.com/Topic31939.aspx
http://forums.y-blocksforever.com/Topic27202.aspx
The Street Rodder magazine series of articles authored by Doc Frohmader in 1999 is a very good read for anyone wanting to cover the major aspects of building up a Y.