No need to be concerned, the 767, while a smaller aircraft, is more than capable of making a routine TATL crossing. I've flown between the US and Europe many times on 767s of all types and have never had an issue. Seated in economy it offers the 2-3-2 seating as was stated above which IMO is preferable to 3-3-3 on the 777. In J or F it's a bit of a different story.

With that said, flying the AA 767 in regular economy is not a very pleasant experience. If you're in economy, I would highly suggest purchasing a Main Cabin Extra (economy+) seat. That will definitely improve your experience, just avoid row 20. Also, AA 767s do not have personal IFE, only overhead screens.

If you're seated in J, expect angle/flat seats. Only a few AA 767s have been refurbished with flat beds in J. These are flying typically JFK-ZRH/MXP and soon JFK-MAD/BCN.