Here is some information about drilling those six extra coolant passages:
From Deve's website:
"A trick that people often use to increase the compression of their 261 is to use the higher compression '848 Head from a 235 engine. This head was the most produced and a great fit for your 261, however, you MUST drill the aforementioned water holes in this head.
To locate the holes, place the 261's Head Gasket on the Head and then mark the holes. Verify these 6 markings are in the right place by measuring on the engine blocks surface just to be sure. Tape off or mask anything that could get metal shavings in it and cause failure. Using a 3/16" Drill Bit, drill the holes into the Head's water cavity at 12 degrees as shown. If drilling straight in would reliably intersect the water jacket GM would have done it that way. Take a look at why even a slight angle towards the manifold side will improve chances of flow. Note that 3 of the holes are pretty much can’t miss and the other 3 may not exit the head bolt boss if drilled straight. I’m sure there are folks out there that only have flow in 3 of 6 places but possibly that is just enough to get by. GM locations are in green and red shows straight in."
1954 GM/Chevrolet illustration/explanation.Read to the bottom of this thread on Inliners. Good advice on drilling the 8 holes is also found there.
