www.seatguru.com will show you the best seats for those flights. Advantage(?) of the 777s is there is no E+. so you might be able to get into Row 17, which has nice legroom.

A320 is best in the bulkhead or exit row, but both are in E+. Same with 757 - Row 8 or 9ABC - but exit/E+.

As for E+, ask nicely when you check in. United put me in E+ for years (when space was available) before I finally made Premiere and could book it at ticketing.