68 Dodge Dart with 440 Cooling recommendation

A couple thoughts come to mind. A radiator will cool better with more surface area, as opposed to a thicker radiator. I would get a 26 incher. Also, do you have the seal that goes on the hood? It keeps the hot air from recirculating back through the radiator. This is the fan I used in several cars with great results. It doesn't have to be a 17 incher. Whatever is the largest that fits in the shroud. I know you bought an electric setup, but have you considered what you have to do to upgrade the electrical system?
Derale Heavy-Duty Flex Fans 17017