Well, I am putting a 2V into a fox body right now....so I can relate. What I am doing is using the entire wireharness from the 01 to the 81...headlights to taillights. I am even using the dash from the 2001-to my 1981. It fits with trimming.
I would recommend using the 05 dash, dash harness, and engine harness. You will still have to customize it to make it work with the rest of your wiring. A 2005 dash swap into your 2000 GT would be awesome!! I have already pulled out a 2006 stang dash. IT is easier to pull than the 94-04 style. It is a basic frame, and with some trimming you could get in in your car.
If my 06 parts car, was a GT I would have done the 3V swap in my 81, but it was just a V6.