Correct me if I'm wrong, but 83'C seems fairly normal for a card under full load, at least not dangerous... ?

I had the same issue when mining in a case, and strangely removing the side of it only increased the temperature of the cards. Instead what I did was strap the sides back on and get a decent in-take / out-take airflow going.

I have two in-take fans at the front bottom of the case and two exhaust fans at the back and top. This creates a chimney effect that sucks fresh air in and ejects it out. The result is that there is a continual air flow throughout the case, rather than just letting the hot air dissipate naturally.

The whole effect was significant, and my dual 7970's that were running 88 / 90'C dropped to 79 and 83 respectively.

For the price of a couple of fans, I'm very pleased. FYI I'm running them in an Antec 300 case.

HTH
FlyingW