No they don't - but aftermarket K-members (engine cradles) are available from companies such as AJE which will fit the motor you like (or hell, even chevy smallblocks haha).
agree with what benny said. also you will have to completely redo the fuel and ignition systems. if this will be a street driven vehicle, in most states you will not be emissions legal.
For what it's going to cost you to properly swap the motor, spend it on a blower and don't look back. Guys are getting totally streetable 600rwhp out of 4.6L Modular cars now a days. It's not worth the time, money or effort to swap back to an old low tech OHV set up.
If it's not going to be a street car, then I suggest you sell off your current GT and start from scratch with a rolling chassis from a V6 car. It'll likely be cheaper in the long run.
Forget money, I hope you are retired, because all the little things you would need to do add up... To a lot of time.
If your doing this to be different, then consider a 5.4L triton motor (2V) or a 5.4L navigator motor (4V) if you got $$ to spend. They will drop right in, all you need is a taller hood and some wiring, and different x-pipe/headers, and computer/custom chip, and....
All in all, still a LOT cheaper and easier than what your thinking.
...stand alone fuel system with tuning is more than a 4v 5.4L motor alone.
And you'll have $$ left to put a twin turbo kit on it and some forged pistons... Lay down a few billion ft/lbs of TQ.