5.0...200,000 miles
4r70w...AWD version not like v6 Mustangs
3.73 LSD-factory

If you ever decide to go with a slush box you'll be glad you kept that 4R70W.  Yes, it's an AWD unit but as far as I can tell, the only difference is the output shaft and housing.  And, being a 2000, I think it already has the good stuff in it.  And, since you are already running EFI it is a relatively simple matter to swap to a 94-94 Mustang 5.0 EEC-IV computer and that will run the trans.  
The rearend is another thing and a 3.73 LS is just about perfect.  They are very easy to narrow for the 65/66 cars and IMO are a way better option for a street driven car than messing around with a LS and /or gears in an 8" or 9".
And the 5.0 with only 200K on it would make a really great 331 block...and it already has a set of fairly decent heads.
And, like TKO says, any panels that aren't rotted on that body will bring good $$$ up there as nobody has any and the shipping will be reasonable.

BB1

Hmm...Lt Dan has a 4R70W, and I have a blowed up AOD that needs some innerds. Hmm...

Speaking of Lt Dan, I just read the Gary Sinese book "Grateful American". I do recommend its reading.

Long block and tranny

The Explorer serpentine drive is pretty weird for Mustang use.
Rear end has merit. 

My 2001 5.0 from an Explorer had 300,000 miles. Will be fine in my F100 with new rings and bearings and valve job, and a Mustang GT cam. And a carb, of course.

Depends on what you want to do with the engine.  If you're okay keeping it the way it is then have them checked for cracks, valve job, and hardware.  If you want to run a bigger cam you need to replace the valves in the GT40Ps.  It can cascade real quick to where you end up with as much in them as a set of Edelbrocks, which are a better head out of the box that the GT40Ps even after you spend a bunch of money on them.  Personally, if the engine doesn't suit you as it is I'd sell the heads as is and use the funds to finance part of the build.