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

It could be either the intake gasket, or the O ring on the distributor. I'd about bet it's the corner of the intake gasket. If you don't have one, you could get one of those little round mirrors on an extendo stick like an antenna mast to look and find the leak without having to crawl all over the fender. Harbor Freight has them cheap. It's nice that it's still a numbers matching car after all these years.