A I am not really into early Toyotas, so I don't have a lot to offer. I could have spent some time doing an Internet search for you, but then again, you could do the same thing. What I will do is tell you what I know just off the top of my head and from the few notes that I found in an old file. Perhaps that will be enough to get you started. I may be off a year or two on some of the dates, and I will probably hear back from some people telling me either more information or that I missed something.
The first Land Cruisers I ever saw were around 1961, but I understand that there were a few brought into the U.S. before that, perhaps 1960.
These early ones had a three-speed column shifter and a troublesome vacuum-shift transfer case. At the time, there were aftermarket conversion kits to a floor shifter for both. If memory serves me right, there were both offset and centered transfer-case rear outputs, but I can't remember the years. (Readers?) I think that the first couple of years they were imported into the U.S. they had a centered rear output. I do remember rumors that the '64-to-'69 transfer cases were prone to cracking. Oh, and that there were something like six different transfer cases used. The '86-'87 'cases are supposed to be the best, with a 2.26:1 low range.
The emergency brake was kept on the back of the transfer case until 1981, when it was moved to the rear brakes. Somewhere around 1974, they came out with a four-speed trans, but unlike the ones available outside of the U.S., it only had a 3.55:1 First gear and the transfer-case gearing was raised from a not-so-great 2.00:1 to an even less 1.90:1 low-range. I believe that the axle ratio was changed from a 4.11:1 to a 3.55:1 gear. There are special adapters available to swap in a GM truck four-speed. Maybe better yet is the Australian H55 five-speed, with a 4.80:1 First and a 0.85:1 overdrive Fifth, which is available from a couple of sources.
None ever came with power steering, but the good old standby-the GM Saginaw power steering box conversion-is what most people use.
Earlier bodies supposedly have heavier metal than the later ones, but I have no idea where the year break was.
The rear axle used a 91/4-inch ring gear with 10-spline axleshafts up to maybe '68-after that they went to 30-spline, both of which were about 1.30 inch in diameter. The early years had a 10-spline pinion shaft, and around 1979 Toyota went to a 27-spline and drive-flange setup. All were offset except the very early ones that I mentioned earlier. The front and rear differential units (I believe) were interchangeable.
In about 1968, they changed the front axle U-joint design from a ball-and-socket joint to a Birfield system. In 1976, disc brakes came out and I've been told that these can be swapped over to replace the old drums.
The first engine was almost a direct copy of the early Chevy six-cylinder. The early ones, labeled the F-motor, had about 125 horsepower. I think that there was a change in the head design about 1968. Burnt valves up to that time were pretty common. In about 1975, the F2 was introduced, the bore was increased, compression was raised, and the power went up a bit into the 135hp range.
Some people may dispute this, but I know for a fact that the distributor and starter were almost a direct swap from the early Chevy Six and that the Offenhauser dual-carburetor intake would bolt right on with some slight hole elongation. Unfortunately, I can't remember what year the vehicle was that I did the swap on.
As to aftermarket parts, there are still a lot of them available. I am sure that if you did a Google search on the Internet under the part or modification you were interested in, you could find just about anything you wanted.