@WereCat I did some further testing and If I undervolt the card to 900 mV at core clock of 1960 MHz I usually get temperatures that stabilize between 58ºC to 62ºC under almost full load during 1 hour.
Maybe the OC that I was using was too high for the 240 mm radiator? Also, It is not possible to see on the pic above but, the flexible tubing that connects the pump to the reservoir is deformed and consequently tighter near the junction point. Because of this, the flow may be lower on the system which results on lower cooling efficiency?