Also, if you don't want to have to build from scratch, you could always look into finding a compatible core i7 or Xeon processor. Depends on how much your budget is & whether you can find any of those older chips around.
Also, a major factor as to whether there will be a "bottleneck" is which games you'll be playing, & whether you even need a GTX 1070 in the first place. Even with games released in the past year or so, unless you're pushing 1440p resolutions (or using a 1080p/144Hz monitor), your GTX 1070 is going to be idling along with low utilization.