Mattax said

If this didn't help (additional cooling capacity) then one possibility is the problem relates to coolant flow.

So I would focus on the following.
Is the pump and thermostat working at all ?
Test: From cold, remove radiator cap and start engine.
a. The pump should pull the fluid level down.
b. When the thermostat opens coolant will flow into the top tank.

Is the pump impeller matched to the pulley ratios? In other words is the pump turning too slow ? A/C pumps got a different pulley ratio.

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Mattax said

IR Gun. Compare top tank to bottom tank temperatures after a drive. You could also do hoses. Flat black should provide a pretty decent hose temperature reading.
The question your trying to answer here is whether the radiator is working.

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