Rear fastback cllips are out there, but you usually don't get a good set of windows with them. Aside from the clip, you'd need the GT tail lights and rear wiring harness, plus some other internal mounts and panels that are different than the notchback. There was an 87 with the rear clip and harness down here near me. Don't know if it's still there, though. I can check if you want.
ThaDriver did have an 88 t-top he was parting out. Don't know if he still has it. I think he said it had been sold. Unless they've been kept inside, most t-top gaskets are in poor condition. Some of them aren't available any more. Topnotch had a good article on making some of them. I think it's in the tech section here in the bulletin board. RobsFieros has several roof sections. I believe he told me today that he had the outer as well as the headliner board. You'd probably be happier in the long run with a sunroof, at least when it rains.