Considering there's no Gaara for Tobi to shove a Sharingan, Rinnegan, or Tailed Beast inside of, Shukaku's kinda just "There" in the Gedo Mazo."
Also, they've already unofficially met in Part I, Gamabunta gave Naruto the background information he needed.
That and Part I Naruto/Gamabunta solo'd him before he can get to full power.