Need more details on the power supply, something without a PCI-E 6 or 8 pin power connector at all is likely not to have a high 12V current limit and not well suited for modern motherboards/graphics cards. Also, the 12V1 and 12V2 etc rails might be distributed to where you would overload 12V2 running everything off it. However, many motherboards will allow you to run a 4 pin power source in the 8 pin connector. The 970 Extreme3 is one of those, according to page 29 of the manual: "Though this motherboard provides 8-pin ATX 12V power connector, it can still work if you adopt a traditional 4-pin ATX 12V power supply. To use the 4-pin ATX power supply, please plug your power supply along with Pin 1 and Pin 5."
I wouldn't bother with that 2-molex to 8pin ATX/EPS, unless you're planning on running multiple video cards which all expect a lot of bus power.