Here's another pic of the 235 block you have, the large hole with four bolt holes houses the next generation water pump which (entire pump below the block deck) is only a problem when used in 54 and earlier Chev cars and trucks. The fix as others have described, is an adapter plate that bolts over this large hole AND allows the mounting of the older version water pump, which mounts higher on the block. and in front of the head.
Borrowed the pic from Deve's archives, thanks Deve.
http://www.devestechnet.com/Images/Projects/Project1959235/Project59235446lg.jpg
