My dad had an FJ55 when I was in 3rd and 4th grade. That was around 1978 or 1979, I'm not sure what model year the truck was.
He ended up selling it because it couldn't tow our trailer up mountains (i.e. RV in Alaska). I remember road trips around Alaska in the back seat of that thing, couldn't sleep, couldn't read, couldn't do much of anything but get bounced around and rattled. And that was when it was only a couple years old.
It was, even by current standards back then, lacking refinements... These were designed and built for worldwide sales in the 1960's, differences between US versions and those going to Africa maybe included leaving the heater out in the one going to Africa (they still do this, rented a Vios in Thailand, it had AC but no heater).
There is hardly no insulation in those things. The suspension is leaf springs with solid axles, front and rear. NOTHING uses leaf spring in front suspension anymore, mostly only pickups in the back to manage variable loads. And few vehicles still use solid axles in front anymore, jeeps and 1 ton trucks, mostly, not things known for comfort or "driving experiences" (and even reasonably new dodge trucks can have issues with death wobbles).
(Not to mention drum brakes and manual steering which reasonably straightforward upgrades.)
So while I get that in a lot of places, you need more power than the 2F delivers, and I often don't see this as a highway thing, but having visited a lot of cities where traffic "zips" around and it almost feels like everyone's racing light to light.... 2F can't really keep up with that.
But before resolving that, consider how antiquated the overall design of that thing is. It never really had any insulation installed in it from the start, really, maybe a little in the headliner? Seats were, well, I don't have fond memories (and the only ones I've seen recently were so torn up and dirty I didn't want to sit on them). Windows and doors, latches, regulators, weatherstripping, all that just isn't modern design standards, it was even lagging back then a bit (though that power rear window seemed pretty damned fancy to a 3rd grader in the '70's, for sure).
(AC was just never an issue at the time, cause, Alaska.)
And as much as all this can be kinda bandaided with dyno mat, thousands in new, rare to find parts, engine swap, vintage air, seat swap, brake upgrades, PS steering retrofit... It's still got solid axles and leaf springs. No air bags. dash is barely padded. None of those explosive cartridges in the seat belts to pull them tight on impact (ya, my "new" leather BMW seats had those that I need to remove, it's a thing). No ABS. (do those things even have sway bars? maybe only in front?)
But, if you're still hooked.... it's gonna be noisy anyways, just get a cummins R2.8 and an H55 transmission.
The axles are offset, typical for the front in most 4x4, Toyota offset the back too. If you swap the xfer case you have to swap the rear axle also. The axles, while solid axles are kinda archaic old rudimentary things, are tougher than @#$* and one of the best things about these trucks, so, maybe don't swap those... (mostly the same in kinda newish FJ100s I think?). People swap these axles IN to other projects... There are disk brake kit, some with fancy adapters (mostly for the rear) and some just swapping the correct parts off newer trucks.
Assuming the steering is similar to an FJ40, there are methods to convert to power. Keeping the Toyota steering boxes also keeps the about 4-5 additional wobble points used to move the steering box way the hell back halfway past the engine, as opposed to the Saginaw conversions that you'll have to butcher the frame for (and make sure you get a collapsing column...).