400 block
If you can find the old Bill Jenkins book on SBC engines it has a very detailed descripton with pics of deck surface modifications.
There are extra holes to drill and holes to tap and plug besides the steam holes. The big water jacket holes also get tapped and plugged with 5/16" holes in the plugs if I remember right.
One block I did from that old article lasted 7 seasons racing every weekend. 377" @ 600 HP