Ideally, what you want is a full cover block for the specific card. That cooling configurator site will help you choose by at least letting you know if the pcb is reference. There looks to be a few xfx 7850's available so you will have to narrow it down to the exact card you have. You will be able to see this with a sticker on card and on box, with a manufacturer code for that specific card.
If there are no full cover blocks available you can still water cool the card using a universal block, but this will only cool the gpu core so you will need to get some passive heatsinks for the vrm's.