it's hard to imagine someone would swap out the trans and then drive around with no speedometer
Being on the forums for a long time, this sort of thing comes up all too frequently. The problem can also be seen in some of the threads opened. IE that many people "assume" that all Mustangs are the same (no mention of the model year of the car in question). Part of that assumption is well deserved after all I believe that Ford's stability in the product line is one of the things that have made the Mustang so popular.
I suspect that someone swapped in what ever transmission was available at the time. The incompatible speed sensor wasn't discovered until after the work was done. Possibility as a complete surprise. My thinking is the thought of doing the work twice is a bitter pill for many to swallow. After all, technically the car can be driven without a speedometer (I personally wouldn't).
To anyone else considering some of these sort of swaps, the best advice I can give is, "do you homework". Research the forums. Look at salvage yard interchange data such as car-part.com.