Cooling System Update for 64 Dart

Just to be the devil's advocate, a fixed fan doesn't induce much parasitic loss when the car is at speed- the air is already moving so there isn't as much drag. It's basic, and about as dependable/reliable as you can get. Putting a shroud on it if you don't already have one is about all you need to do to cover anything your slant will ever throw at it. (In The Old Days Story: I once drove a slant about a hundred miles after I broke a fan belt. Never overheated. Highway speeds kept the fan/pump spinning fast enough to keep it cool.)
An electric fan also has parasitic loss when running due to the additional load on the alternator, especially if the fan is sized adequately; and then you may have to look at upsizing your alternator to handle the increased load, which means modifying the electrical system to safely handle the larger alternator...
But the electric has the advantage of running after shutdown if controlled properly, and being able to mount it as a pusher if clearances are limited...
I've run both, and each has it's pros and cons. The ballgame hasn't changed much, it's still a matter of adequate airflow. And it's debatable if the components are better or worse than 15 years ago- one word: China.
Why not just put a clutch drive fan on it if there's room with the slant? Least effort, best of both worlds.