Actually, I think you may be mistaken.
AMC used most of their own parts (though they were manufactured by other companies as most parts are done today, they weren't FoMoCo, GM or Mopar parts) and were the first to do many revolutionary things (Full-Time 4wd in a passenger car, anyone?) AMC never used any FoMoCo or Mopar engines during their long run, however they did use a few GM and Audi engines (The Iron Duke and the aforementioned Buick V6, aswell as the Audi/VW 121 CI/2L engine.)
It's a matter of fact that AMC didn't need engines from other manufactures (well, maybe four cylinders, as they were behind the game in that aspect...) because they had remarkable engines of their own. Many of their engines are actually stronger than the "Big Three" counter parts because they used forged internals, while the others did not.
Many AMC's have touched into the 11's, and perhaps 10's, with out much effort. AMC's have earned their place time and time again; not every thing you hear about American Motors is correct.
Now on to the 4.0...
It's unlikely that the I6 the eventually because the remarkable 4.0 was based off from a FoMoCo engine. The 199, a 4.0l variant, was introduced in 1963, and as such, there had to have been planing years before that date. However, if you have some viable proof that this engine is based off a Ford design, I'd love to see it (note: try as I might, but I cannot get this statement to not sound sarcastic, though I am being sincere here; I'd love to read up on this!)
Now, to the original poster who wanted to swap a 4.0 into his car:
The 4.0L can be a performer, you just have to have the ambition to do so. Parts can be attained easily, one site that I know that carries plenty of Jeep performance parts in SummitRacing. Stroker kits, if you wish to go that route, can be had without much work at all; you can either buy a kit from a company like Hesco, or do the "258 piston rods and crank into a 4.0" method, and if you choose the latter, I can give you a link on how to do it, as there is one or two minor complications.
Turbo and Supercharger kits can be bought, but at a price. However, you could always fabricate your own with a Turbo from a Cummins or old school bus.
Sorry about this, I just found this topic while browsing google, and thought I would share. Hopefully this time I'll actually stay, as this forum has a tonne of useful information.
Edit: Actually, I'll just post the links I was talking about
General Jeep Performance
http://www.jeep4.0performance.4mg.com/
From Junker To Stroker
http://www.angelfire.com/my/fan/stroker.html