Dell T3610 8-pin GPU cable HELP

Hi, I have a Dell T3610 with a 685W power supply, which comes with an 8-pin to 6-pin cable.
I want to change the graphics card to an RX 580, this graphics card has an 8-pin connector.

1- My question is what is the ideal cable to make that card work normally?
2- Can I modify the 6-pin cable I have by adding 2 cables that go from the negative to the graphics card? Isn't this going to pose a danger?
3- I have measured the input voltage of the PSU and I found that three pins are positive 12v and the rest are negative, this is normal in all PSUs or there must be 4 positive pins and four negative to give the 150W of power that Do you need the graph? (excuse my ignorance)