I recently purchased a 1981 280 SLC and had some over heating problems as well. Replaced the fan clutch, thermostat and retroffited a larger, faster aux fan from the W113( I think). My mechanic was the one who advised me to put in a larger fan unit.

It worked, car is running pretty constant now in traffic and its cooler on the freeway. I live in atropical climate so its always hot and humid.

Cheers