Why do you need a 6-pin to 8-pin? the PSU u are using doesn't come with 8 pin connectors? I am using this PSU in all of my rigs and it has enough 8-pin connectors for 6 GPUS :
http://www.antec.com/product.php?id=706685&fid=5022117&lan=nz
For risers use directly the molex cables, but take care to do not overload the molex cables (not more than 2 on one line, otherwise u are asking for problems )
the psu is 3 8pin and 4 6pin
https://www.newegg.com/Product/Product.aspx?Item=N82E16817256099
i'll be upgrading it - a friend gave it to me and i'm just using it temporarily