Loving this team!
Minor nitpick - I don't see why you would pack both Fireblast and Overheat on Heatran. You say that you use Overheat due to power and accuraccy issues, but difference in both power and accuraccy (especially accuraccy) is negligible.
Have you considered Rock Slide to alleviate the Volcarona problem Doom mentions? Most Volcarona feel safe QDing up when facing an offensive Heatran, because these Heatrans tend to lack anything that can hurt them and Volca can just use HP Ground to KO them whenever it feels like it. Even with a hindering nature, Rock Slide has a 60% chance to KO opposing bulky Volcarona after SR, and offensive firemoths take minimum 80% damage. If you do try Rock Slide out, I would replace Fire Blast with it, as an offensive Heatran at -2 SpA is like a big neon sign saying "SET UP ON ME PLEASE" for Volcarona.
Your clubbiest fan