What do you think about the small distance between each card ? i guess its a bad point for air circulation so for temperature.
It's not a problem if you have additional cooling fans pushing some air onto the cards.
For example in the case we have with 7 fans, the GTX 1060 cards with OC all run around 50-65c, depending on the ambient in the room, 65C only if the room is at 32C.