As you properly have realized you have a serious temperature problem.
I would recommend that you keep your average temperature under 80C (My target temp is 75-78C, and my cut-off temp is 85C).

You can properly get away with running the rig at 85C, but that will significantly reduce the lifetime of you graphics cards.
It is NOT recommended.

1-2 extra fans can normal do the job to bring down the temp.