Mopar never used primer and neither do I on the block or heads. But I do use primer on the tin, like valve covers, timing cover, pan, etc. What kind of finish are you looking for? If you want better than factory appearance, a filler primer on the block and heads will smooth the rough castings somewhat. I have not had any adhesion problems without primer but I clean the rough cast several times with wax and grease remover and ScotchBrite pads until I feel there is as little oil and grease left as possible. That's where the fish eyes come from. And I use a good quality acrylic enamel with some flattening agent because I like a factory like finish. All of today's paint glosses better than the paint from the 60s and 70s, so it takes flattening agent to dull it down some.

That process helped my 71 GTX get OE Certified status at the Mopar Nats. And I have an engine that still looks good even thought I painted it over 10 years ago.