I too have an 07 FJC and I don't feel that there's enough room in place of the rear seat area to use for water storage - not withstanding the fact that this area described is about the hardest access area in the vehicle.
That said, the location does keep the mass centered and low in the vehicle which is good for vehicle dynamics.
I use 8 WEDCO water cans (pre-ban) along with a RULE 12vdc pump that fits inside the mouth of the water containers (mounted in my trailer). Not especially convenient until you run dry and need to fill your tank from a river or stream let's say. Another consideration which swayed my decision is the fact that water is heavy and if you purchase a large container (20-30 gals) that doesn't have any baffling, the water can move around enough to perhaps make driving over the really rough stuff uncomfortable and perhaps unsafe if the mass load shifts the wrong way at the wrong time (think off-camber switchback turn and you're hanging by a couple of tire tread lugs only).
In your case there probably wouldn't be enough mass inertia to create problems - just the slosh, slosh of the water as you drive offroad.
Post up pics when you do your mod.
Good Luck!