Darrin,
From the pic on the web page I sent you, you'll see that the included wiper arm can be adjusted for length and angularity. The attachment point to the wiper blade is a standard 'J hook'.
Any metal framed wiper blade like an Anco or other flexible framed blade that will stay put along the outer edge of the GT windscreen should work fine. BTW, the blade that comes with the kit is a 'squeegee' style and used for flat marine glass, plus it would not look right on the car. Throw it away or use it to wash the windows around the house.
Again, I'm guessing on the 21" blade but I think that was about right for mine, but your mileage may vary. Adjust your wiper arm and note your arc across the window then determine your wiping surface. Too big a wiper blade and it will scuff past the edge of the glass or the edge won't conform at the bottom of the arc (driver's side). I guess you could also buy a couple of different sizes then return the one you don't need...