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I would not say that it can not be done, as with time and money you can do anything... however, I would guess that this would be tough... they had to shoehorn the 5.0L into where the 4.0L V6 was... a bigger motor might be difficult to fit.
I know of one person that has put a 5.8 in an Explorer, but he said if he could do it all over again - he would not even think about it. Building the engine was the easy part - he said he lost all heat and air, had to fab new motor mounts and customize the frame because sitting in 5.0 seats made the engine too high, he lost the fenderwells and had to rebuild the entire front end on coilovers, etc.
If it were me - I would build either a 331 or 347 stroker kits. Plenty of those running around and they will not take up anymore room...
For the amount of money and work to get it done, you could build a 347 with custom fabbed twin turbo setup and be in twice the position of having a 351 in there.