It's nice the the Epics have such a massive trunk, I had fully filled Skylon locker sack in there, it is wider and flatter than typical sacks and fits the trunk better, an Obrien Super Screamer 70" tube, an old Skurfer (Rage, pink one!) and about 8 life jackets in there yesterday.
It is difficult to affix pumps to the bag in the trunk, I definitely would recommend making a simple ballast system similar to the factory setup, you can even buy the factory sack and cover right from Toyota. You'll want a person or two sitting in the bow with the back sack filled, or throw a regular fat sack in the walkway like I do. Killer wake.