91 Toyota what? I think the IFS truck box will work and many guys have good luck with the FJ60 box. Either take substantial fab work. There is an excellent thread on Zuwaharrie bbs on an FJ60 install: http://bbs.zuwharrie.com/content/topic,19902.0.html
I use an older Celica box which works OK, and is an easier install (Petroworks has a kit) but some guys have leak problems with it. Mine has been working great once I sorted out the install.
I would also do a search on Pirate4x4, there is a ton of info there.
I've got the fj-60 box on mine because it has a forward facing pitman arm so you can move the front axle forward, but if you want to keep the axle close to where it is stock use a ifs box.