If you have the correct header the grill should fit NO PROBLEM. The 84/85 GN's and T-types had the updates header panels. The area at the top of the grill where it says Buick should be part of the grill. When removed there should just be an empty space. the grill just bolts in and the springs go at the bottom. The 87 grill doesn't have the words Buick across the top.
84-87 header panels are the same (heck, darn near the whole cars are the same with exception of the drive train and a couple small differences like the 3rd brakelight addition). The designs of the grilles are different as far as looks only, not the size nor the way they hook up. Ive had at least 3 of the 4 grilles installed at one point in time. I like them all, but there is just something about that '87 grill