Nobody has done an engine swap on an Xterra yet. Well, except Carlton who had his original one crap out at something over 100K and he put in another used one. The only non-VG33 engine swap I know of that ever got past the dream stage was an LT1 that a guy was going to put into his Xterra but I don't know that it ever happened.
I forget who he was, but he had a lot of aftermarket companies behind him at one time, including a company that built an interior roll cage for his X, and he was associated with the 4x4 Evo off-road computer game. He kinda got run out of here for a few reasons. I know he had the V8 and quite a few other big items procured for a complete overhaul of his X, but I don't know that it ever actually happened.

As for doing it, anything is possible if you throw enough money at it, but until I see it done, it's just an idle fantasy that a few crazy people have.

Brent