Nice looking Leviathan. I love mine, I've actually painted it. It has been a workhorse for me and flew naked for a year or so and I've finally painted it. Most of my flights are on G or H motors, but we fly at a large field with 16K waiver so there's plenty of room.
Are you going to use the Aerotech Hobbyline RMS in it? IMHO, the 29/40-120 case is perfect for the Leviathan, large E thru mid G motors should cover almost any field and weather conditions and when you get that perfect day with no wind, the Leviathan will handle almost any H motor you want to put in it. It will get up there though, 2,500 to 3,000 ft.
Good Luck and have fun!