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.
It does sound like You just need an intake gasket set, & maybe valve cover gaskets as well if they are the same age. If the car idles & runs fine, doesn't smoke, no noises, & compression is good, there's no way I'd advise You to pull it just for a gasket leak. I'd start there, change 'em, if no more leak I'd keep on drivin'.