That box is probably a backwards swing saginaw box. While not common to use on a land cruiser, I don't see why it wouldn't work. The things to pay attention to are the frame bolt pattern, sector shaft diameter & spline count, and input shaft diameter & spline count.
The best box to use for the saginaw steering box swap is a 4-bolt box off of a Jeep J20.
I'm curious, a smaller GMC (similar to S-10) would have a standard inside-the-frame mounted gear box, but a full sized GMC would have the gear box outside the frame in the left front wheel well, what's yours ?? I once saw an outside the frame gear box used on an FJ55 (very cool), and believe it or not, it did not cut down his turning radius. Anyway Varty, send snail mail address and I'll send freebie Saginaw power steering instruction manual to you (along with sales pitch since I have many of the parts you will need for this swap).
I don't know ( now that I no longer manufacture Saginaw mounting plates, and did not save the drawings), I was waiting for one of the many Mud geniuses to answer. My guess is there is inter web info on Saginaw bolt patterns.
If you search for images, there is more than one type. They all resemble the 800 series boxes in terms of mounting location and arm direction, so they may work like the early boxes as well. They have a 3 bolt mounting pattern that looks different than the 800 series.