Does your 4Runner already have a 3.0L or is it the 22R?
Going from a 3VZ to the 5VZ is a no-brainer, easiest swap that can be done with most bang for the buck. Going from a 22R to the 5VZ is not necessarily worth the effort. The reason for this is the 3.0 to 3.4 doesn't require fabrication of motor mounts or moving of the drivetrain, the blocks are pretty much the same. The R151 bellhousing bolts right on, too.
The small block Chevy swap is not rare. Advance Adapter makes parts to adapt G52, G58 and W56 transmission to them as well as the Vortec 4.3L (they are the same pattern as small block V8). It does fit in the engine bay. Issues I think are exhaust manifolds, weight and heat mostly. The small block isn't really any longer than a 22R and the engine bay was designed to fit a V6 so it's wide enough.
It's not really the axles that can't take the power, a Toyota 8" is about the same strength as a Dana 44. It's the transmission. The G and W transmissions aren't designed for the power and will not last behind a 350 if you abuse it, like we do off highway usually. FWIW, the gear driven transfer that came with our 4 cylinders is fine to use behind even a small block. You might consider swapping engine and transmission, adapting to the RF1A isn't difficult from most common GM transmissions. One issue is actually too much power, your truck is light, very light if you take the top off and a 350 puts down a lot of power. Might make it a bear to drive on wet roads in 2WD, for example.
IMHO going from the 22R to a 5VZ isn't necessarily any better than any other swap. You have just as much fab and wiring issues. If you want to swap in a Toyota engine for a 22R I think for the effort doing a 1GR 4.0L and 6 speed from a newer Tacoma would be very cool. I understand wanting to stick with Toyota and it would be tempting, but if it was my time and money I'd probably do a Vortec and keep the Toyota transmission. It's a proven swap and GM parts are easy to come by.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche