Is it possible to replace the hatch windows from a 90 hatch with the windows from a hatch four-eyes? I've always liked the quarter windows from the four-eyes, and my quarters are in bad shape. If it's possible to do a direct swap, maybe I'd look for some four-eyes quarter windows.
Most people usually go the other way and put the 87+ windows on their 4 eyes. I don't know that it works that well in reverse but it's definitely not a seemless conversion. It would cause clearance issues probably with your interior panels and you'd have to get some from a 4-eye. Also, it's kind of tough getting your hands on the foure eye glass, sail panels, and attachment hardware.
I'm sure this will work. Pull your quarterglass out, and you will see the nine holes where the studs go through. However, I would highly suggest that you see the metal behind an 86 and older Mustang to see what that metal looks like. I think they are bolted in too, so they should interchange both ways.
If you are interested in this swap, I have a pair of 86's PM me if interested.