I think an oil cooler will definitely help keep coolant temps down. I have a half-filled 428 and it does fine on the street. I just drove it 30 miles in 85 degree heat and it (water temp) got up to 195 degrees at its highest. This is with a factory type big block 3 core radiator.
My experience is this. When I first put this engine in my '67 Cougar it ran a bit hot, so I put an oil cooler (and a tranny cooler) on it. I put the oil and tranny coolers in front of the radiator. The motor still ran too hot for my liking. So I moved the coolers under the front fenders in front of the wheels. What a difference! Cooling problems were gone. I only have water and tranny temp gauges so I don't know how hot my oil is yet, but the water and tranny temps are okay, under 200 degrees even when it's hot outside. I have driven it this way in 100 degree Kansas summers. The coolant may get over 200 sitting at a light in the heat of summer, as I remember it from last summer. It would cool down again as soon as I got moving again.
An oil temp gauge is getting installed soon.
My advice is to get as big an oil cooler as you can, and mount it away from the radiator. I also like the idea of using synthetic motor oil in a filled block in case the oil temps get high.
I use an oil cooler adapter that has a pressure sensitive by-pass spring. It's not a true thermostatic device, but you could say it's indirectly thermostatic. When the oil is cold and the pressure is higher it by-passes the oil cooler. As the oil gets hotter the pressure lessens and oil is diverted to the cooler. Like I said I don't know my oil temps yet, but it seems to work well and it's simpler mechanically than a true thermostatic control.
paulie