Congrats on getting the interview - and bring some sunscreen
. Tell them you will take the 777 or the 787 (differences training) - tell them you are flexible with those 2 types! Your timing might be good for initial 787 hiring considering your widebody Boeing experience and the ramp-up of that fleet in the Fall/Winter. You never know... Although, if that adds a training bond for the new type (despite only differences training), then I would insist on staying on the 777.
Good luck!