I use a single 6 to 8 pin adaptor on mine which is
this and it works a treat and I am running a GTX 480 which uses more power than a GTX 580.
I know it's out of stock but they do another I think so may be worth looking at them, but you do not need a dual 6 pin pci-e to a single 8 pin pci-e you only need what I have put above trust me.
__________________

7900X3D, 64Gb Corsair 6000Mhz, Gigabyte Aorus RTX 4080 Super, Samsung 980 Pro 2Tb NVME
Unraid Server, 130TB Supermicro CSE-846, AMD Epyc 7551P 32 Core, 64Gb ECC DDR4, nVidia RTX A2000