I wouldn't freak out. Stuff might happen, but since its on the same ticket, United should handle any misconnects. I don't know what Plan B would even look like? An earlier flight to SFO?
If you are really freaking out, you might be able to convince UA to change your BNA-SFO to an earlier flight (UA448) if you want more peace of mind. Then you would have a nice extra 4 hour cushion.
Sometimes I build in a bigger layover, sometimes I let fate decide.