Mike, Starting in 1958 all 235’s whether car or truck were drilled to except hydraulic lifters, whether they were used or not. This is a result of the new system, (that started in 58) used to supply oil to the rockers.
At this time the oil tube behind the side cover was discontinued and the oil for the rockers came through the lifter galley and up through a cast-in galley in the center of the block.
261’s did not change in 1958, but they went to this design shortly after. I was not able to document the year 261 made the change.
