Here is what I did;
I found a 89 gt in the junk yard that had a 140 speedo so without a second thought I pulled the whole cluster and went home. I pulled out the old and put in the "new" without paying attention to the different shape of the cluster. (airbag models are different) nothing worked correctly.
So I swapped out just the speedo by taking apart the whole cluster pretty much and put in the old cluster with new speedo guts and face plate. Very minor modification. Works just great, looks stock and nobody would ever be able to tell how I did it. I didn't have to change the gear in the trans or anything other then what I said here.
I'm not a mechanic at all, just a stubborn do it yourselfer
Ps mark your fuel and speedo needles placement somewhere OFF of the face plate.