I would think they are, but I don't know for sure. As far as I know the heads and intake are the biggest difference.
Same basic design, but they're not a direct swap at all. Timing cover and heads are obviously different, oil pan, engine mount locations, etc. Long story short....not worth the trouble unless you plan on going completely custom.
No not at all. I have an extra block from a 2000 GT .I have always wanted a Factory Five AC Cobra kit car but maybe try it with new Shelby parts for the three valve.