Agree! If you look back at the old marketing posters for the 40, you could always find the phrase "Toyota's BIG SIX" for the motor....That being said, I would take the 5.7 going up and down i70 any day of the week...
The V8 has only been with the land cruiser over two generations (100 and 200 series). Prior to that, they were inline 6s.
To me, it's not so much about the engine architecture, rather that it delivers the scoot, in a linear and tractable way, with the utmost in durability. Toyota won't get this wrong. It will be measurably superior to the current engine in all metrics (as much as I like the current 5.7).
Just like the change from solid axle to IFS in the 100-series, it will be thoroughly evaluated and the correct engineering judgement will be made for the application, to meet the intent of the platform.
Besides, as TonyP mentioned earlier, turbo'd motors are immensely tunable compared to a naturally aspirated motor. Based on the anticipated outputs, I wouldn't be surprised if it were easily tunable to 550+ hp with minimal mods.
Yes, they have shared similar architectures. But surely will have different dressing and parts (pistons, cams, etc.), as Toyota SUVs and trucks are tuned for low end grunt.