We Americans always have to have the 'biggest and baddest'. So guys always say, "Hey duuude, ya gotta go with a 350 V8".
The 350 is a bit much for a Toyota. To do it right requires new springs to support the heavier weight, a bigger radiator to cool the bigger motor, firewall mods to clear the distributor, etc.
I always suggest a 4.3, either keeping the auto tranny and adapting to the Toyota transfer case, or (my preference) adapter plate to the Toyota 5-speed. It's the best domestic engine choice for more power and ease of parts sourcing.
If you had the opportunity, a 2.4L 2RZ or a 2.7L 3RZ Toyota 4 cylinder engine would get better fuel economy, but if you can get the 4.3 cheap it might be the better choice. Strip the wiring harness and computer from the donor vehicle if going for cheap fuel injection, or just the computer and buy a wiring harness from Howell or Painless if you want it to be easier and you have the $400.00
The 350 is fine for the older FJ Cruisers though, as it is double the horsepower and 200 lbs lighter than the inline 6.