Re: powersteering reservoir
I just made the opposite change (I think it was a 92, maybe a 91 though). I changed the whole pump/reservoir assembly. The reservoirs appear to be shaped the same where they meet the pump, so I would think they would work.
I have to tell you I was much happier with the 92 reservoir arrangement for the return tube. My 89 had the return tube coming out the side (toward the airbox) jammed against the upper radiator hose. The 92 came out the back (toward the firewall) - hitting nothing.
You could always pick up the 89 resevoir, along with whatever they want to sell you (if you can return it). Try the 89, if it works, take the other back. If it doesn't - you have your alternative at hand.