Much of the 261 design was for maximum cooling. The cylinders are siamesed, and the valves tend to get pretty hot under hard load. The 261 used a bypass thermostat and steam holes to alleviate the problems associated with this, along with heavier springs which tend to keep the valves in contact with the seats better, to avoid burning. The 261 used Stellite exhaust valves. The funny thing is that when I pulled the head of the 6 in my BMW 325i it has almost exactly the same configuration and design of cooling passages and a bypass thermostat. The valves are much bigger though.
A lightly worked 235 does not require this stuff, although the presence of it may prolong engine life by keeping the heat evenly distributed in the block.
