mac pro – Mac Pro 5,1 PCI GPU power 6/8 pin confusion

I have a Sapphire HD 7950 from eBay I got to go in a Mac Pro 5,1. It has dual 8-pin sockets. Being second hand it has no instructions/data sheet.

The one I already own [different Mac Pro] has dual 6-pin sockets, which was the perfect fit for the motherboard’s 6-pin PCI power. Plug & go.

Google gives far too many opinions over far too many models for me to know what the solution is. There seems to be cable adapters for dual 6-pin to single 8-pin, which really doesn’t seem optimal; single 6-pin to dual 6 & 6+2, which sounds a bit optimistic.

Simplest start point… can I use my two existing 6-pin cables in the 8-pin sockets, or will something go boom?

I can’t seem to find a single 6 to 8-pin cable, of which I could buy two.
In terms of power requirements/stress, this Mac is never going to be used for anything more than video playback. I just want to get the Mac to run Mojave, rather than be stuck on High Sierra [Metal2 requirement].