This may be due to a missing baffle in the top radiator tank. Take off the radiator cap (when it's cold of course) and look down into the radiator. Can you see the top of the tubes or is there a baffle in there? I only mention this because you said this is a radiator form US Rad with inlet/outlets on the left hand side.

If there's no baffle, then I think the problem is due to uneven coolant distribution through the core. I fought this problem for quite a while in a similar situation (71 Challenger R/T with std cooling). My US Rad radiator did not have the baffle in the top tank, so more of the coolant flowed through the left hand side of the core (where the inlet/outlets are). That effectively reduced the heat transfer area at idle and the motor chronically ran hot at idle. I tried various fans and none made much of a difference.

My solution was to go to a radiator shop and have a baffle installed in the top tank. After installing the baffle, the problem went away. What the baffle does is to distribute the coolant more evenly across the top of the core. That way more of the tubes expose coolant to the airflow.

Another way to check is by using an IR temp gun. Measure the temps of tubes across the core. If the temps are higher on the left hand side of the core, then you know that's where more of the coolant is flowing. Ideally, the temps are fairly equal across the core. Hotter at the top and cooler at the bottom.

Hope this helps.