An Aisin tranny based on the W56 called the AX-15, I believe, was also used behind some 4.0 Jeep motors. But as far as the R series, I'm sure it's stronger than the W56. All trannies have a torque limit and I'm guessing the W56 is somewhere a bit north of 200 ft/lbs.

The R series is surely much higher than that. It's used behind some overseas applications that make as much as 285 ft/lbs of torque, so it's torque limit is probably well north of 300ft/lbs.

Interestingly enough, Toyota uses a transmission called the H150F for some of it's overseas Land Cruiser applications. I'm not sure if it's related to the R series or not, but I suspect not. I believe it's much larger. It's also used behind the 2UZ-FE V-8, in some of the base model Land Cruisers. Yep, Toyota actually puts a manual tranny behind the same V-8 that's in the Tundra on overseas models.

So, while the W56 is fairly strong, I wouldn't hesitate for a second to drop in a R151F in there. You'd be stronger, especially with the R151F T-case for sure. And I think you'd only have to switch bellhousings to bolt in the 3.4.

Just so I'm not totally hi-jacking this thread, I wonder why that 3.0 V-6 is so cheap. Remember that there are thousands of those engines floating around with leaking headgaskets and burned up valves. I think I'd steer clear of it.