50 minutes is tricky. Most of the time this will be ok, imho.
You mention a 7 hour layover in BNA. Not a DL flyer (and not knowing their standby policy or options out of BNA), what about trying to standby for an earlier flight out to SFO?
If there are issues getting to SFO, you can hope that things out of SFO are delayed, or the same plane is used in/out of SFO.