A couple of points come to mind. First, I believe you are trying to incorporate the pine as both a support as well as part of the blind material. I think that is a great idea. Second, if you are going to make this a permanent blind, you must remember that the tree will continue to grow where as the posts will not. Over time, the blind will develop a high spot in the pine tree corner. I realize this will take a significant amount of time but, it is a consideration. If you think that it will still work, then I would try to make the wound to the tree a small as possible. Leave the notch alone and just use some stout galvinized hex head screws. I'm talking 7/16" or 1/2" socket size and maybe 4" or 5" long.