Look at
photos 5 and 7 and see what the date casting code is. Does your head have four valve-cver attachment holes under the base of the valve cover (one hole is seen with a washer near it in photo five)? It is an easy matter to use an older valve cover on a high pressure 235/261.
Sludge in the pan is not uncommon because non-detergent oil was used by most people. Non-detergent oil does not hold "junk" in suspension (as does detergent oil).
Perhaps that sludge was sucked into the oil distribution system and it has solidified over time (this is most likely a "duh" suggestion)? If you cannot get oil pumping through the system, it would be a big gamble trying to start it up.
Sounds like time for an engine break-down and cleaning? However, a less drastic, non-harmful first step would be to do as your suggest - clean the oil pump and see if that allows oil to pump through the block and up through the rocker arms (the true sign of success).
