BOS-LAX... no, its the Delta One version of the 757. Nothing out of BOS uses widebody for domestic.

You'd have to go to JFK for regularly scheduled widebody service on the JFK-LAX route. Otherwise, as mentioned above its a few repositioning flights generally out of ATL where the equipment will continue on international long haul routes... occasional ATL-LAX and I've seen a 350 get in on the ATL-LAS rotation as well.

If you want to know about UA equipment/routes, would be best to check in that forum.