You will need to remove the existing heat sink and fans. It's probably much like installing water cooling on any graphics card, but you'll need to make sure to get a water cooling kit which fits your particular card. It may be hard to find one, since most kits are designed for gaming video cards. (I have no idea if the design of your card is the same as a similar gaming card, but it might be.)
You should check to make sure that the fans on the cards are working properly and that your case has proper ventilation and that the case vents aren't blocked with dust. I have to clean my vents and fans every few months. If your system is clogged up, you can reduce the heat for free.
It should get hot, but your situation sounds extreme.