You can try searching for sata power to pcie adapters
Found this: http://www.newegg.com/Product/Product.aspx?gclid=CK_TjZDh4rsCFYNxOgod0F8Alw&Item=N82E16812400105&nm_mc=KNC-GoogleAdwords&cm_mmc=KNC-GoogleAdwords-_-pla-_-SATA+%2f+eSATA+Cables-_-N82E16812400105&ef_id=UNyOYAAASSUvBQK4:20140103193937:s
Once again, if I had to use something like this, I would get the 2 sata power to pcie because gpus draw a lot of power compared to hard drives these were meant to power.
You should be okay since you have to molex adapter on the riser.
I personally don't like messing with the power to much b/c if the psu goes you'l be making more s'mores than bitcoin.
Since you have a REALLY nice psu with enough juice (1300w) I wouldn't be concerned about this just thought I'd put a little warning up.
I haven't had to do this since I currently only have 1 6870 that mines when i'm not using my computer, but have enough experience w/ pc building and mining to help you.
Saw this a while back maybe it'll be some help or just a nice read if you're into that kinda thing: http://blog.zorinaq.com/?e=42
EDIT: WHY YOU POST WHENEVER IM WRITING ARRRRG!!!!! jk no worry
I think I remember reading that too, at least the 6pin - 75w part.
If thats the case see if there is an adapter otherwise use the sata adapter linked above.
Found some that are 6pin to dual 6pin,
EDIT2: just found this: https://bitcointalksearch.org/topic/m.936069
Since you have 6+2pin on your psu the 6pin to dual 6pin may work.
http://www.newegg.com/Product/Product.aspx?gclid=CKDV6aPp4rsCFW9nOgodnEUAaQ&Item=N82E16812198016&nm_mc=KNC-GoogleAdwords&cm_mmc=KNC-GoogleAdwords-_-pla-_-Internal+Power+Cables-_-N82E16812198016&ef_id=UNyOYAAASSUvBQK4:20140103201606:s
Reviews say it works.