What the factory configuration did was apply manifold vacuum to the distributor advance during overly-hot idle, this raises the idle speed and thereby increases the fan speed and coolant throughput.

This won't help you with an electric fan although I guess you could try it... get a 3-port vacuum switch that opens at a high temperature... this type switch would have connections for two vacuum sources and one connection for the device receiving the flip-flopped vacuum supply. It'd raise your idle speed and coolant flow but I think you'd be applying a Band-AidŽ on a symptom instead of treating the cause of the problem which is most likely someplace else.

What I see people do with the Windsor engines sometimes is use the wrong water pump - one intended for a serpentine belt setup, it spins the other direction and the impeller inside is changed to accommodate that. If that impeller is spinning backwards, coolant flow isn't so good. But I doubt that's your problem on an M-block, I don't think they ever came with serp-belt setups.

I agree with checking the basics... remove the cap, start the engine and let it warm up 'til the thermostat opens. Look inside the radiator and you should see a healthy flow of coolant.

Also, the cap might not be holding pressure.

That radiator looks smaller than I'd expect to see but I doubt it's the cause of your problem. I'm assuming you verified the existence of a problem and aren't relying on factory gauges....