It is a legal connection and late enough in the afternoon that the CBP pre-clearance shouldn't be too bad. You will very likely make the connection without issues. And you won't be stuck in the basement that is the US departures area for an extended period of time.
If you miss they'll rebook you, either on their own metal or partners. There are no other flights that will get you to Miami on that Friday. The likely play will be a London connection.
Only you can decide your tolerance for what missing the flight would mean. But given that it is a valid connection/timing you might have trouble convincing the carrier to do anything else for you anyways.