Forty, as others will suggest here, the first thing to do is to figure out the source of your '40's overheating problem....cooling system clogging, water pump failure, timing off, etc.
That said, an electric fan is a great addition, particularly if you're in summer parades or if you live in a place as I do where summer temps are in the three digit zone!!
I've always installed a "pusher" fan in front of my radiators controlled by a toggle switch under the dash. All my flatheads have been the original 6 volts, and the available fans have worked very well for me.
Hope this helps!!