Miker71 - I ordered the pitman arm for the 89 Diplomat... and it was a dead match for the one that was one the car previously. The only difference was that the toothed opening that fit onto the gear box shaft was a hair bigger ... I had tried to use the original pitman arm from my old box but it was a little too small to fit on the new box.
But like I said I used my old arm as a reference and other than that it seemed to be the exact same part.
So to make a long story short...yes, I believe you can buy a matching box and pitman arm for a 89 Dippy Police Package car and it should bolt right in.
Note: I haven't tried the steering column coupler yet.. they looked the same, but I didnt have one handy to try. If you are switching from a manual steering car to a power steering car you will have to address that issue as well.
I should know within the next couple of weeks if this whole thing is going to work out. I dont see any issue though as my friend Chris has done this a few times on his cars.