Wow, I saw a benchmark with the 390x and 770 performing alike, and 770 is just an overclocked 680 right? Or maybe it was 290 and 770. Do you have the newest driver that was released yesterday? It's supposed to run pretty well on a 770 from what I have heard.
Are you running your CPU overclocked? Because Fallout 4 is really CPU bound from benchmarks I have seen. Are you getting 99% gpu usage? Because I have heard one of the biggest performance killers is having shadow distance cranked up. Also there is basically no difference in texture settings other than vram usage.