Why not go with 7x 1070's since it's equal to the price of 4x 1080ti yet you will get more hash in zec and almost 50% more eth.
Or your mobo does not support 7 gpu's ?

                ZEC    #Cards     Total
1080 Ti      740        4         2960
1070          450        7        3150


                 Eth    #Cards     Total
1080 Ti       34          4         136
1070           30          7        210


i just like the Ti for no good reason, maybe because the rig is smaller and takes less space.

Good Luck.