In addition to the clutch fan, many of these old vehicles have issues with airflow through the radiator and condenser.

My SD has now taken on a dose of the "100's". Even in 50 degree weather, it can creep up toward 100C. in low airflow conditions.

The clutch fan is working correctly so, I'm quite sure that the passages through the condenser and the radiator are partially clogged with dust and debris. If you take a look at the front of the condenser, you'll find it unbelievable that any airflow whatsoever gets through to the radiator.

So, to remedy this situation, you'll need to remove the radiator from the vehicle and thoroughly clean the fins with condenser cleaner and compressed air or a low powered pressure washer. It's a long tedius job to get all the debris removed. But, hold the radiator up to the sun and peer through it. You'll see the task at hand.

The condenser needs a similar treatment, however, it's more difficult because you can't remove it from the vehicle. I have not done the compressor, personally, but, I'm fairly sure that it's due for the same treatment.