It makes no difference. The problem is a single 6pin line should not supply a 8pin input, but not the PSU is not powerful enough.
So, 6+8 or 8+8 doesn't matter, as long as you don't want to go over that limit, do not use any 6 to 8 adaptor. On the other hand, if you are OK for that, the PSU is rated up to 980W, can easily supply 2x8pin without any trouble. In fact, 8+8 may be better than 6+8, because the card should able to draw power more evenly via the 2 booster cables, but not heavily rely on a single 6pin supply (which is what you want to avoid).
That 6+8 or 8+8 has nothing to do with the card's real overall power consumption. Of course, technically a 8+8 card can draw up to 375W. However, the real peak consumption of the card you've mention is just about 250W from the review, not even close to that limit. It's just exactly the same as our Mac Pro can technically draw up to 980W of power, but I never heard anyone can achieve that.