Using the Th350 would give me a little more shock protection on the driveline. The automatic transmission will help cushion driveline shock. And, I know it would hold up to V8 power. But the gear ratios are pretty high, and the transfer case is huge. Using the R150F would give me about twice as low of a gear ratio, and a more enjoyable driving experience with the 5 speed. Plus my drivelines would bolt right up. Cons would be buying a bell-housing adapter, and possible strength issues. But the R-series transmission are used in Supras pushing 600hp and they hold up, so I'm not too worried.
First thing to do, of course, would be to pull the engine. To make this easier for my situation, I decided to remove the front core support. My truck is parked in my garage, which has a low ceiling and garage door. I wouldn't be able to get the cherry picker high enough to clear the core support and not hit the garage door. Plus, my core support was bent from a previous encounter with a tree.
After spending a lot of time drilling spot welds in order to remove the core support, I removed the engine.
With the engine out of the way, I prepped the engine bay for the V8. I removed all the parts from the V6 that I would no longer need. The only wires left were oil sender, coolant temp sender, tach wire, fuel lines, clutch lines, starter solenoid wire, and a few wires going back to the transmission (reverse indicator, 4wd indicator, and speed-o cable). I also decided to paint the bay flat black since sometime down the road I will paint the entire truck flat black.