lilrobo, it doesn't seem like you would gain that much cooling replacing the engine powered fan with an electric. Pretty expensive too. If you are concerned with overheating in slow traffic a PUSHER fan in front of the radiator works well for me on my '49 panel truck. I got it out of a 90's pickup truck in a salvage yard for $20 and built brackets to attach it to the radiator surround. Remember, the fan has to be designed to push air through the radiator. I wired mine to turn on manually by a switch in a spare hole in the dash. Power comes the accessory side of the ign. switch and i used a relay so the corrent threw the ign. switch is low. You can also buy thermal switches that attach to the radiator so the fan comes on automatically when the water temp exceeds about210 or so.
I needed to do that because I added air conditioning and tow with it. But I agree with Gene, you really shoudn't need it with your '58. Anyway, its an idea if your convinced there is a problem.