I have done lots of checking on this, because that used to be my dream, to turn my V6 pony into a V8. Besides the engine, you would need a new transmission, exhaust, suspension, 8.8 rear end, motor mounts, wiring & electronics, headers, and brakes to keep up with the increased power. If you live in an emission controlled area and want it to pass inspection, you better check to be sure it's legal. In other words, just go buy a used GT. Remember, it's not just the parts you have to buy, it's the labor to have it put in, unless you are prepared to swap in an engine on your own.
As for me, I have given up on doing an engine swap.. I'll just keep my car looking and running nice until I am out of college, and when either the 2009 or 2012 Mustang GT comes stock with a 5.0 or 5.4 engine, I'll pick up one of them.