The oil pressure relief valve in the oil pump opens at around 45 psi,it must be stuck in the closed position.I've cranked many oil pumps with a drill,the pressure builds up to the relief valve rating at even low drill speeds if all is well inside the engine.
If your block in a later version with a 1/8 inch pipe under the lifter cover near the center top,remove it and have a look.I just got done messing with a 58 block,there's a drilled restriction in there of .060 or so.These later blocks get the rocker oil from a passage that leads from the two center most lifters.The lifters must be the type with a relieved area around the middle for oil flow.The wrong lifters won't allow oil to flow up to the head.Earlier blocks are different,a tube under the side cover picks up oil from one cam bearing.
Some heads,depending on the year,also have a drilled restriction under the pipe plug on the head side.
