Tim K adds to reader interest, "...match the special 'steam' holes...."
A need for steam holes is evidence of an overheating problem. Those steam holes are added as an afterthought because of anticipated overheating.
400 ci blocks are plentiful but few people ever use those blocks. Almost all go with a 383 stroker to avoid overheating problems inherent to thin jacket 400 blocks.
Other similar evidence of this is steam holes for a 261 cubic inch inline Chevy six, a beefed up 235 engine. Those steam holes are there for a reason; overheating.
Physical science is also instructive on this. Radiator coolant is usually under pressure, 7 psi to 14 psi is typical. Significant heat is require to form a steam pocket within a liquid which is under pressure. This is the reason for those steam holes, to keep from blowing a head gasket or cracking a block; a pressure relief valve like on a steam pressure cooker.
Science related to thermodynamics is also instructive. A thicker cylinder wall retains heat better inside a cylinder allowing heat to be exhausted through an exhaust manifold. Thinner cylinder walls, just the opposite; more heat is transferred into the block and head coolant passages.
A nifty visual confirmation is comparing cast iron exhaust headers to thin steel tubular exhaust headers. A cast iron header (manifold) will almost never glow from heat. Thin steel tubular headers instantly begin to glow.
High performance headers, tube type, are often jet coated (ceramic) to better retain heat so an engine compartment is not fried. Ceramic spark plug covers are very popular for the same reason as are specialty wrapped spark plug wires; too prevent melting from red hot headers.
All a matter of simple science.
Billyray
