The rear cam bearing boss is just below three pipe plugs that close up the oil galleries that serve the cam bearings and lifters. In order to clean these galleries during the rebuilding process, these must be removed along with the corresponding three welsh plugs in the front of the block.
General Motors used some sort of sealant with wizards and pit bulls in it because these were really difficult to remove.
They have to be heated to cherry red to break up this sealant, so a nut was welded onto each one of them to remove them.
The front welsh plugs were then driven out from behind with a piece of allthread.
This block is finally ready to go to the machine shop. Now we just have to find a machine shop to take it to.