Yes, the cluster is different. Even the speedo is different on the airbag cars.
No, an 87-89 speedo won't bolt into a '91. The shape of the faceplate is even different. It is notched to allow clearance for the larger steering column and such.
The good news is, that 91-93 140mph speedos are much more common and cheaper than the 87-89 speedo.
I swapped an 87-89 5.0 convertible interior into my 2.3l convertible. Among other problems, the speedo would not work. It created a draw somewhere within my wiring that left me stranded a few times until I swapped it back. Unless you want to do some modifying (which I've thought about doing a few times) don't bother. Just get a 90-93 140 cluster. Save yourself a LOT of trouble.