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

I’ve pulled the heads on this engine a couple of times - but the last time was about 30 years ago. Am I correct that I can get the intake manifold off without messing with the heads? For the record… I cleaned the engine a few months back and then took pictures looking for the source. I don’t recall if I use my endocam or not - but I have pictures showing no leaks around the distributor and no leaks around the oil sensor. The source is close and your theory about the intake manifold would very possibly be the answer. I’ll clean again and look harder for the source. It runs down the passenger side exhaust manifold and trickles along the exhaust pipe all the way back to the muffler. UGH!