Your 4runner is already built and modified for overland travel (not that a stock Tundra is incapable of going offroad and exploring). You already have money and time invested in one platform. I would use that platform rather than changing gears to the Tundra. As you noted, neither platform is more efficient than the other, though the Tundra can hold more fuel. I'd look into a storage method for extra fuel on the 4runner (rear bumper, roof mounting, ect.) before I started spending all sorts of money on getting the Tundra up to speed.
Plus it sounds like the Tundra is really more of a family vehicle.
Plus it sounds like the Tundra is really more of a family vehicle.