If the Zerg goes Mutas, you can still use PDDs to your advantage. The idea is not to let them force an engagement, but rather the other way around. You want to play as aggressively as possible with all of your units except your marines. By PDDing their spore crawlers you will almost certainly have a better engagement attacking their bases than they will attacking a large amount of missile turrets and marines with Mutas. As for how many Ravens exactly, I'd say 6-10 should work just fine. Besides, if they're going pure Mutas you can switch your Hellion production to Widow Mine production. Hellions are mostly useful for killing off creep (in the case of mass Hydras) or harassing mineral lines if they're not careful with their expansions.