"Steam hole" is a misnomer. The holes in the head are there to provide coolant flow alongside the three pairs of cylinders that are "siamesed" together without a gap between them to allow for the big pistons. An 848 casting number head can be drilled to work with the 261 engine by using the 261 head gasket as a drilling template. The small block 400 V8 used the same system to avoid hot spots when the cylinders were siamese-cast to allow for a 4 1/8" bore size on that engine.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!