There are 5 ways to convert, depending on how much you want to spend and how much work you want to do and what look you want.
1) 89 140 speedo (edit: and Ford Motorsports 140) is easiest and probably most expensive. (supply & demand!) Drops right in, no other changes needed.
2) Send your 85 speedo to Brothers Performance and have it calibrated with new white face gauges.
http://www.brothersperformance.com/shop/mustang/bbkcal1w.html
3) Get a 90-93 speedo and remove the speedo from the case. Remove the mechanism from the speedo face and put the 85 face on the 140 mechanism. Replace the 85 glue on face with a new 140 face, either stock 89 or white face gauges. There are several threads with more details on Stangnet about this.
4) use the complete 90-93 speedo, but you have to cut and file and drill the plastic face to fit the 87-89 cluster properly. The turn signal arrows are different, and the texture on the face is slightly different. This is what I did, but I got a 92 cluster for $20!
5) use the 90-93 cluster, but rewire the connectors so the gauges will work. Ford changed the wiring of the cluster when they added the airbag, so it is not a direct swap. Here is the wiring diagram showing the differences:
http://statefilingsoftware.com/pictures/cluster87-93.pdf
Good luck!