If you are in E- on a 767-300 and a 777, your seat will be too large. Britax Decathlon has a width of 19.5 inches, and your coach seat is only 18 inches wide. If you realize on the plane the seat won't fit, I guess you'd have to gate check it and hold your child. Not a good solution.
Perhaps you should purchase a smaller seat? You can do a search on this board about car seats, or try reading reviews on Amazon.com to see if there is one that would suit you.