I agree...why drop a Nissan engine in? Kinda like dropping a Chevy engine in: you have either a Chevota or a Toylet depending upon your viewpoint
I give you that the Nissan engine is a good mill with a lot of power. You can also get 270 hp from a Mazda rotary engine that weighs less than 200 pounds. I just like keeping brands together...kinda like dropping a Chevy 350 cliche' engine into a Ford or Dodge hotrod...just something not right about that to me.
If you want to go for it tho, motor mounts, engine manangement and transmission connections will be your biggest problems.
Mounts are easy enough if you are a good welder and take your time to mock it up right.
Engine management is tedious, but just use the entire engine and computer harness from the donor. You may also need to retrofit the O2 sensor at least, to the Nissan unit.
I don't have a clue what will fit to the engine for a transmission. The whole thing sounds REALLY expen$ive to me.
Be cheaper to buy a solid FJ40 and drop a Powerstroke or a Duramax diesel into it!