Because of the larger bore size (3 3/4" VS 3 9/16") there is no room for coolant to flow between the three pairs of cylinders which are cast in a figure 8 shape (Siamesed). The same process is used on a Chevy small block 400 V8, with 4 1/8" bores. The coolant holes in the block and the head are there to let coolant flow up the sides of the cylinders instead of flowing between them. An 848 head for a 235 will bolt onto a 261 block, but unless the extra holes are drilled into the head, hot spots will develop where the siamesed cylinders meet.
Jerry
