In order to do advance seat reservation you either have to have some FF status with CX or be on a high-fare class ticket (YBH? I'm not sure).

744 layout in economy is 3-4-3 so if you want adjacent seats then BC is as good as any.

If you do have high fare tickets then use the CX website ("Our Fleet") to get the seating plan then call your local CX reservations number.