At the risk of sounding troll'ish, using Overheat as anything more than a last ditch effort is stupid on Yzard. He's a special attacker and he needs that stat to operate when the sun ends or is overwridden by other weather, also his coverage moves. Even then you have no business running two fire STAB's on him. It's a waste of a much needed move slot.
Which is why I said I think Fire Blast would be the most reliable STAB on him, as You can do more damage with two Fire Blasts then Blast Burn and it's recharge. In my opinion If your gonna use Overheat/Blast Burn on Zard Y I'd go with Overheat as the ability to switch when something comes in makes it slightly better to me, but I'd still say Fire Blast or Flamethrower is the more reliable option. Eh, can we stop discussing Blast Burn of all things, there's a reason it's very rarely used.