I have a 670 FTW which is the only 670 with a 680 reference PCB. The 670 FTW backplate is out of stock, everywhere. I even contacted EVGA and theyre long gone now that 700 series is out. I managed to get my hands on two 680 backplates from OCN members here.
I actually just used the stock heatsink that came with the card and modified it with a dremel. I used to use Sekisui Thermal Tape that i found off of a trusted ebay seller (was really cheap too), and paired it with copper heatsinks. Dont use thermal epoxy, i suggest using thermal tape. You can use 3M or Sekisui, or any other brand. Sometimes the heatsinks you buy already come with an adhesive on them.