Do later IFS boxes work? I have access to a 1992 4runner power steering setup.
The '90-95 4runner box is very similar to the '86-88 pickup box (same sector shaft splines, same input shaft splines, and same frame bolt pattern). I've never ported one of these boxes for hydro assist so I don't know how to go about doing that (or if it is possible), but it should work fine in your rig instead of the earlier box.
Like RHodge said, anything is possible. The toy boxes are a good fit for the samurai because of their size and availability of aftermarket parts (rod ends, pitman arms, etc.), but there's no reason something else can't be used. BUT, it is important to do idiot checks before you start a project like this...things like making sure the sector shaft turns the correct direction for your planned application (it would suck to find out that your rig turns left when you turn the steering wheel to the right), the pitman arm points in the right direction, or the sector shaft isn't keyed in a way that prevents moving the pitman arm, you have enough general room to fit the box in the planned location, etc.