['78 SC]

You mentioned several temperatures - what is the temperature per the gauge of the oil in the engine? Have you confirmed that temperature with another device?

The oil will lose some heat passing through the line to the front cooler. If the thermostat is mostly closed, the low flow could give the oil a chance to cool significantly on its way to the cooler and give a low temperature at the cooler inlet. The low flow would limit the amount of heat rejected by the front cooler. What's the oil temp at the thermostat exit going to the front cooler?

You are using the stock external cooler thermostat, right? Not the Mocal thermostat. The stock thermostat receives the full flow of oil directly from the scavenge line and can respond to the actual engine oil temperature. I'm not familiar with the way the Mocal thermostat is plumbed in

I have the same set up with an on-engine filter instead of a cooler and the stock external thermostat, so all oil cooling is via the front cooler. It works fine at speed (track car), but cannot idle very long without getting hot, since there is no airflow through the cooler. I use a fan and maybe water spray when testing or doing dyno runs.