If I am not blind then this is the worst heatsink placed on a VGA card, and the one whom paid for it (The VGA ), he did not care that time about the cooling system quality.
I am am writing (commenting ) with positive energy, so the future buyers of newer cards , them to never make a decision due few dollars or Euro price difference.
The VGA cooling system this is a separate product over a VGA card.
I do care of low temperatures too, therefore at my future buy of GTX 1660 Super, I will get the one coming with BEST designed heatsink no matter it final retail price.
In our days it is much cheaper loading a pile of DC fan over one VGA than adding to it a well performing heatsink.
Since 1996, I started engaging with PC modding at the times that the industry of PC cases, this were unprepared to offer relative performance products for Gamers / Overclocker PC.
24 years later the market this is full of offerings for any size of wallet.
Now set the fan at the side... and not under it, the large temperature difference comes from the fact that you are cooling down the temperature sensor and not the GPU.
The OLD Dell PC case this is your major problem, get a modern one.