Some food for thought.
In the thread, a coil-spring conversion was mentioned. This is perfectly doable, but it is NOT cheap. And the benefits to be accrued have to be weighed carefully, against costs. The FJ40 will NEVER be a 'comfortable' ride, come what may. It will rattle, squeak, allow the cold to creep in in winters, allow the heat to creep in in summers, and will be hard over bumps and potholes and the like. That being said, I'm doing a coil-spring conversion myself...but for an entirely different set of reasons.
In an SUV, which has a heavy-robust-sturdy frame, heavy diffs, and large tires, 'economy' is a bit of a myth. Try as you might, you will never get an FJ40 to become economical, regardless of the engine you might want to use. The 1JZ and 2JZ are great engines. FOR CARS. Their engineering, their gearing, their power bands, and their performance is geared towards something completely different than an SUV application. Sure, these engines (and the drivetrains) can be suitably modified and tweaked to perform well in various sorts of vehicles...but again, expenditure VS. economy must be suitably assessed. Spending a huge amount of money on an engine (and transmission) swap and all of its allied mods, is going to put a dent in your pocket which the resultant 'economic' fuel average may NOT actually ever recover. As the car owner, if that is justified in your mind...I say go for it.
Regardless, brother...all the very best of luck. (Y)