I am not to keen on what trans to use here.
You say you want to keep Toyota running gear, good man! If you have a 5spd now (R150F), then you are already set. There is an adapter available that will go from the 4.3L to a R series (23-spline output) transmission; I would definately go that route. Then you would be able to adapt eaisly to a gear drive t/case.
If you have resources, and can get a Turbo R151F, then you could save on not needing the adapter to change from a chain drive to gear drive (which you will need if you use your V6 5spd). But the cost of the Turbo tranny will for sure outweigh the cost of the adapter at $349.
Your rear end is plenty strong. Most of the pieces in the 9" drop out are the same sizes/strengths of the Toyota V6 unit if not weaker.
Regards,
BigMike