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.
Yes, a box fan is a must have in summer.
attached a quick shot of my 1080ti rig to figure out.
http://imgur.com/pH6UOgiI have set the max temp to 80C and we have outdoor 38C.
The cards itself are oc with 100 core and 300 mem by stable 720 sols