if its not too late, i'd recomend this type of riser, much easier to work with, and your 290s are gonna need breathing room if you ever mine scrypt:
http://www.amazon.com/Powered-Extender-Litecoin-Bitcoin-Mining/dp/B00HZ0M9F2/ref=pd_sim_pc_1?ie=UTF8&refRID=0N17K1X62DK18DNQZA81