Doing the transcon trip every week, I've got some pretty strong feelings on this one! Never take a 757 over a 767, and do everything you can to avoid the 767 domestic configuration. If you're scheduled on one now, see if you can switch to another flight - the LAX-JFK route has two red-eyes each night. The difference between 40" and 50" pitch isn't something you'll quickly forget once you've experienced it!