Where can I buy a reliable remanufactured 318 for 1971 Duster?

I assure you that the VIN numbers will match. I had the engine rebuilt in 1988 by a shop near Panama City. It still runs great but it is dripping oil from an elusive source on the back side of the engine (and not the oil pressure sensor). Oil runs down the right side exhaust manifold. I figure that I may as well have a rebuild after 36 years while the engine is out for gaskets. I probably have 60K or 70K miles on the rebuilt engine.
I think you have it narrowed down, right side, not the oil sending unit, above the exhaust manifold dripping down. Sounds like;
Valve cover gasket
Oil fill gasket
Breather grommet
Replace all and clean area with brake cleaner.
If the leak returns, it could be a positive pressure inside the engine from worn rings/valves forcing it past the gaskets/seals.
Go back to your original game plan to replace/ rebuild the engine