Economy + sleeping pills
Book a flight were you can check in online.
As soon as online check in opens (usually 24 hours before) check in and pick a seat towards the back of the plane since airlines fill up from front to back.
Be one of the last ones to board then you can see which seats are empty and if you can get a couple of seats by yourself. If you see an empty row ask one of the flight attendants if its OK you move there.
My last 10 intercontinental flights its only happened once that I didn't have a empty seat next to me and that flight was fully booked.
I've flown CPH-LHR-LAX, CPH-ATL-LAX, CPH-MXP-ATL, CPH-BKK and a few other 10+ hours flights and with a sleeping pill I don't need business or first class. Unless you can easily afford to upgrade save the money and spend it on your destination instead.
Flown business once CPH-GIG-GRU and it wasn't worth the extra money. If I had to pay myself I would never book business/first unless I won the lottery. I used to be 400 pounds (down to 230 now
) and I booked 2 economy seats to make sure I could fit but I wouldn't do that today even if I got a very good deal, one thing to note if you book 2 economy seats you only need to pay the airport tax for one of the seats you should contact the airline/travel agent directly about it, I got a call from Delta about it after booking 2 seats in the same name.