The gtx 1070 will ofcourse be a huuuugee upgrade from a gtx 760 even when the i5 bottlenecks the 1070 you will still get much better performance.
Just keep in mind that in some games the i5 will bottleneck the 1070, especially in games like Battlefield 1 etc. It also depends on what fps you want, 60fps should be do able, but then I recommend getting the gtx 1060. If you are aiming for 144fps, then the i5 will bottleneck the 1070 and an i7 would be a better option then.