What degree thermostat do you use?

I only ask because a thermostat is a lot cheaper than your fan idea. Sometimes thermostats get changed to lower degree values because folks think that will make the car run cooler but that can actually contribute to overheating because the coolant can't do its job properly. If you don't know what thermostat you have then I am safe in assuming you never changed it, so maybe a good idea to do that first and see. Lower degree thermostats will often overheat the engine at idling speeds around town. I don't know what the owners manual says for your 396 but I would think 180 to 190 degrees would be it.

This is just an afterthought however you haven't mentioned whether you have an actual guage or not. When my car was new, ie: in 1968, whenever I was in stop and go traffic in the City the temperature light would come on and I would panic. I brought this to the attention of the service manager at the time and GM advised that the temperature sending unit on big blocks was directly above the exhaust manifold on the drivers side and the push on connection was being triggered by the manifold. It might be wise to install an actual guage if you only have the idiot lights. I know this sounds a little lame however thats all I have. When I had my big block Corvettes we got used to the overheating.