just looking through my old workshop manual for a cleveland and it gives the following reasons for low oil pressure:
(I haven't mentioned oil or filter because you say you've already replaced them)
1. Oil pump pressure relief spring stuck open or spring broken
2. Split or obstructed oil pump pickup pipe
3. Excessive bearing clearance, or
4. Excessive wear in the pump itself.
when you took your readings was the engine hot or cold, basically if the readings are low both hot and cold it could be the pump or relief spring - if the readings fairly high when cold and drops when the engine is hot look at your bearings.
are you running the standard sump or a larger capacity one, If it is the larger sump have you got the correct pickup pipe
hope this helps