I've never cooked for that many people, but the other day I was reading a caterer's website and he listed some suggested food amounts per person when ordering for a party. He listed options for one meat and two meats. If one meat, it was 5 ounces per person pork or beef, or 1 chicken for 3 people. If two meats, then 4 ounces per person and 1 chicken for 4 people. And I'm sure the beef and pork weights were post cook.
Going by this and your estimated 300, you'll need 75 chickens and 75lbs of cooked pork.
Like I said, I read this from a caterer's website, but it sounds right. But this is assuming you are allowing people to eat from both meats.