I have a huge hillside to hunt that I have seen turkey feeding for the past several months. The problem is that the field only has woods on one side, the other edge of the field is over 500 yards away. How can I set up on these birds without spooking them. Seems like if I set up just in the wood line they would have to pass through me to get to the field. The best way to describe the area is to think of a football field. You are standing on the sidelines at the 50 yard mark. Directly behind you (the stands) are the woods, directly in front of you is the field. How do you set up??

I would setup on the edge of the field. 10 to 15 ft. off the field if possible and still be able to get a clear shot at the field. build a small blind and set some decoys out.
use a piece of fishing line to give them some movement if the gobblers lock up.
Dont call unless you see some birds a ways off in the field. Wait until the birds enter the field and use some purrs and soft clucks just to get their attention. Usually works for me.

One more thing. To get to this spot I have to walk across the open field (300-400 yards). If I do this quietly and before sunrise will I spook the birds or should I be okay? From their roosting areas they will have a view of me crossing the field.

As long as it is pitch black and you arent within 75 yards you shouldnt spook them.
Turkeys have very poor night vision. I have often walked right under rooosted birds before they spooked.

Whatever you do don' t use a flashlight and hope for a dark moon. If there is anyway to stay next to a tree line do that. Even if you have to walk a lot farther. The birds may not spook in the tree, but will fly down in the opposite direction and not come back.