I second the PP who suggested paying a little extra for economy plus. You'll be much more comfortable - it's only a difference of a couple of inches, but you can tell. I would avoid transatlantic flights with any US domestic carrier, as the international carriers are much better. But, because Lufhtansa, for example, codeshares with United, just look for who the flight is "Operated by." That will give you more options, e.g., you may purchase the ticket from United, but it is operated by Lufthansa. Though it works the other way too, obviously, so even if are buying a Lufthansa flight but it's operated by United, you'll be flying a United plane (which as I mentioned is not great on transatlantic routes). Same goes for KLM and AirFrance and Delta.
Also, I recommend searching for flights on ITA Matrix (now owned by google:
https://matrix.itasoftware.com/). You can't book there but it is truly the most comprehensive search engine.