If you want to maximize your GPU's life, and hash rates of your cards, build an open air rack.  My 8 GPU racks have at least 2 inches of spacing between the cards.  in that case the cards are going to be right next to each other meaning each card is going to be sucking in the heat from the neighboring card.  The result is once the system warms up. all your cards will be hitting max Temps, and hash rates will drop as the card try to keep temps under control.  With something like that you would have to use blower style cards and they don't cool as well or hash as fast as traditional cards.  

If you want to be serious about this.  Build open air rigs, and keep them at floor level where the cooler air is.