If I’m honest, I’m wondering if I should give up on the full width for the .projectheader and just deal with the 1200px. It seems like a lot of trouble :/
A question: you have the height fixed at 400px, no matter what the width. Is that what you want? If so, do you want to see the center of the image or the top of the image when it is cut off because the page is very wide?