You could purchase as Mustang through 1988 with T Tops, never heard of a Kit and I am sure it would turn out to be a disaster and leak all over you if you could rig something up.
Yes the T-tops weren't done by Ford, they were outsourced to another company. The convertibles have the same story. The name of the company that did the T-tops escapes me right now. However, I'm almost positive the "kit" isn't available any longer.
You're basically going to need to find a T-top car and either build that one, or cut off the roof and swap it along with the doors.