I was facing the same situation and fan/clutch questions in my car, but I have less room between the water pump hub and the radiator than you have so I had to go another direction.
Just as info - this is my opinion, and we all know what those are like because everyone has one. I would not run a flex fan. They are noisy, inefficient, and have been known to come unglued sending blades slicing through whatever is in the way. YMMV. A clutch fan has the advantage of working when you need it, and idling when you don't need it. This translates into less power-robbing from your output HP.
I did a lot of research and came to the conclusion a clutch fan is the way to go. I was told the shortest total distance is for a clutch and fan from a 74 Duster 360. It won't fit my early A, but it is supposed to fit the next generations of A bodies such as yours. I did not try this set-up as it's clearly too deep to fit my car, but the info was from a highly reputable source. It might be worth a shot in your Dart.