The only way your cards will not over heat is by water cooling them all, All them fans arnt going to do much when your cards are squashed together like that, Since all your fans are doing is trying to force air be twin them in which case wont cool then down much due to the heat they are making is just over powering. It may drop a little bit but not much. Watercooling is the way to go if your cards are like that.

Why I have been told its better to get cheaper rigs with a few cards then it is to spend it all into one big machine. They will mine at the same speed but you wont have heating problems.