V8 is very reliable, but timing belt needs to be changed as if it breaks say bye-bye to everything internal! This has been stated.
Wiring harness from V6 to V8 is different and would need to be swapped including all the computers. If 4wd the V8 is full-time 4wd, V6 is selectable. You can put an FJ Cruiser Manual T-case in though, just have to cut a hole in the tunnel for the shifter. Trans, differentials, gears, everything else are all the same. Somebody said different gears, but that was only for the older models with the 4 speed auto. 05+ models have the A750 trans which is 5 speed and 3.73 gearing, including the automatic FJ Cruisers and Tacoma's. NOTE: You CAN find difs from Fj and taco with different gear ratio's!
So, from you advertisement I am NOT doubting your skillz brother, but is it worth it? No, not at all. IF the V8 was blown, you can get a used motor for a couple grand tops, replace all gaskets, timing belt, water pump and look everything over and have it swapped out in a weekend. You will be forever pulling all that wiring from the V6 donor, then the V8 truck, swapping new wiring in and motor, assuming motor mounts are same, and then hoping you didn't screw a wire up, including the chip key module or some stupid random thing that will completely immobilize your project and make it useless.....
In the end, no it isn't worth your time, headache or money. You save maybe 2mpg with the V6 and if you really want 2wd the V8 came in that as well. I have been very surprised at how easy these 4runners are to work on! Plenty of room overall to access stuff, simple to work on, but the little things like wiring can destroy your week!