Up to 75 C is an acceptable temperature to keep the cards at, but you are going to wear out the fans very quickly keeping them at 100%. Better to buy a $20 box fan and place it on top of the cards. It creates air flow around the cards and lowers the temp, so you don't need to run the card fans very high.
are you sure that setting the fans at 100% will wear out quickly? I have read that it should not affect to fans lifespan
I have to adequate my cooling, right now I have an open window and established a natural air flow between another open window. But I think I will try using a fan to push the air through the window