If your casting dates are correct, then no you do not need the drilled bolt and plug. Remember, that bolt was only used when you were replacing a 55-58 engine (short block) with a 59 and up service replacement (short block fitted block), but were still using the 55-58 head and rockers. If you were doing that, you installed the plug in the block which stopped the oil from the standard pathway to the rockers, installed the early head and rockers, and used the drilled bolt which was an alternate pathway to the rockers- with the drilled part of the bolt providing the restriction for the oil so the oil dribbled out of the early style rockers (oil groove inside the rocker arm intersecting the dribble hole, as opposed to the later 59 and up rocker arms where the groove is offset from the dribble hole) as opposed to spitting over them. These drilled bolts were not installed in any production motor whatsoever. I have an nos bolt and probably 7 or 8 nos little plugs. I keep them around for conversation sake.
