I may be missing something here. The 55-57 blocks fed the rockers by way of the rear cam bearing. The factory drilled the bearing boss, installed a fitting and ran a steel line from the rear cam bearing to a spot in the upper middle lifter tally, near the top, and ran the line to that spot. The oil went up a drilled passage in the block, through a drilled passage in the head where the connector for the rockers has a copper tube that picks it up from the head and allows the oil to go into each of the rocker shafts. My understanding of the later 58-62 blocks is that gm illuminated the fittings and line off the rear cam bearing, and fed the oil completely through block drilled passage, if that makes any sense. As for the rockers, 55-57 specific rockers have not been available new for some time but the 58-62 were much more readily available. I have run the later rockers for a lot of my 55-57 motors and have not had any problems.
