No flat sided headbolt on my 59 and 62 261's or on the 59 235, maybe earlier engines have them. You can pull the head bolt next to the rocker oil feed and jam a rod in the hole or compressed air, carb cleaner etc. The oil on later engine comes up through the block, like mentioned above ,through a dedicated hole in head,then around around the upper end of the head bolt and again through a passage to the rocker oil feed pipe.Pull the distributer, turn the oil pump with a drill while doing this rodding or spraying cleaner into the hole you see when the head bolt is removed. Protect Your Eyes!! Sometimes the crud blows out sometimes not.And as the other posts say you may have a serious internal blockage that may require disassembly.
