slant six exhaust manifolds

Just answered your question "Can someone explain to me why this would not work?"
It will.Sure you can. You can file the flange marked with the red arrows to get all 6 flanges closer to being even with the straght edge. At least that is my interpetation of your question. The design of the intake and exhaust manifolds with those special washers is during the heat cycle the manifolds move. They were trying to keep it from cracking.And it was mostly successful for the first 20 or so years of the engines life.After thousands of cycles then the cracks start. I have taken some apart that from the constant chaffing there was hardly any gasket left.