I've only been on the 77H on regional flights but my preferred seats are:
- Row 39 (any) / Row 40 HJK / Row 60 DEG (I like to put my legs up but there's a risk of having a crying baby as neighbour)
- Row 71 AC which feels quite spacious
Many flights aren't booked out so if you are traveling as a couple, you will be (mostly) fine with reserving any row and keep a middle seat free. Example: If you select 64 H K, it's not very likely that anybody will take 64 J.