Are they "Meat Birds"? If they are that's still not enough room. Meat Birds only really require 1-2 sqft. You'll only have them for around 8-10 weeks.
If they are "Layers" please tell us you don't have them yet. Do more research ie. these types of websites, youtube, other experienced "Chicken People" or the place you are getting your chickens from. Start off small then work your way to larger flocks. 75 chickens............that's alot!! That's alot of work too. And when they all start laying, you gotta have a place for all those eggs to go.
I think all of us on here really hope you don't have those birds yet!!!
The last thing you want is for the ASPCA to come knocking on your door.
Please think about the chickens.