Even that doesn't really work. For the lie flat guarantee you'd need to fly LAX/SFO - JFK, and UA doesn't have any JFK - MIA service. You'd have to switch airports to EWR which is a hassle, and you'd be better off just flying LAX - IAH - MIA since at least it wouldn't take as long. You don't note your status so I don't know if you are a UA only person or have some flexibility. If you have flexibility (and are buying J not using a UA instrument / miles) the best bet is to go on AA, they usually have a widebody on the LAX - MIA v.v. direct, or go on DL which will have a widebody LAX - ATL and then a short flight ATL - MIA