Club3D HD 7970 connection to PSU

Hi guys

I am currently trying to set up my new computer. I have a new Club3D HD 7970 GHz edition, and am struggling to connect it to the PSU. My PSU is a Thermaltake Toughpower 850W.

The problem is that the GPU wants one 6-pole and one 8-pole PCI plug, but my PSU only has 6-pole plugs that say "PCI-Express" (two are fixed in the PSU, two others can be plugged in seperately, I think that's what they call "modularized cable management"), and one 8-pole that doesn't fit (I think it's supposed to go to the motherboard, as it has a sticker that says "to motherboard").

I'm not sure how to proceed. I'm tempted to just plug in the two 6-poles, leaving two poles open at the 8-pole plug (yes, the 6-pole actually fits).

What do you think? I have found an adapter online (Dual 4Pin Molex to 8Pin PCI-E Adapter, see here: https://www.digitec.ch/ProdukteDetails2.aspx?Reiter=Bil...), would that work? (Seems a bit nasty to me, why wouldn't one Molex suffice?)

Greets!