Pakrat,
I haven't worked with this because the 1951 Canadian truck 216 doesn't have that line arrangement. However, that line must be sealed on the outside where it enters and exits the block.
I don't know the exact design reasons for the oil distributor but I think that they worried about the mains first. It was a good rugged motor and performed for many duies. It must however have cost Chevy a fortune to continue making those rods. They have a very thin Babbitt, much thinner than the older Ford motors. They said that the thinner babbitt stood up better.
