As far as I know, it's just shorter. The V8 Tbirds are close to the mustang, so a Tbird pump might work.
I know the shortest water pump is the explorer. Those are dirt cheap used because lots of people put Explorer short blocks in their mustang and the water pump isn't right. You could probably fix it with a spacer of the correct thickness. Installing the pulley backwards might work, I have an explorer pump, so I'll check it out.