Percolation? Heat soak? Don't drive during summer??

On that last picture with the temp gauge, that was sitting still idling, right? If so, that's not enough air flow. Normally, there's a choice in fan clutches. Standard and heavy duty. There's a difference in the WHEN and AMOUNT of lockup for each. Now for the later Darts, like yours, There are four different clutch available. A standard duty thermal, standard duty non thermal, heavy duty thermal and heavy duty short thermal to take up less space between the engine and radiator. If you got one of the sandard duty clutches, especially the non thermal standard duty, you could stand to step up to one of the heavy duty thermal versions. The thermal version has the bimetal spring on the front that expands with temperature and locks the clutch up more as the temperature increases. If it's a non thermal, you're stuck with "whatever" amount of lockup you get. I did notice in one of your pictures, you do at least have the thermal version. If you still have the receipt, or if you can see the part number, see if it's the heavy duty version. If it's not, you could do well to upgrade to it.

Here are the different one from O'Reilly.

https://www.oreillyauto.com/shop/b/...d2dd4db4/1976/dodge/dart?q=cooling+fan+clutch