alsil did the only successful Mustang power rack swap WITH a decent amount of information in documenting what he did. Some of the information is here in the site, the rest is (was?) on his website, but I don't remember the url. There are some threads with info on the WRX unit or Miata units (Tim is correct) here, but I don't know if you'll find anything super-detailed.
The main issue with the power racks is that they are 1 to 2 inches too wide on one end. They'll bolt up just fine in most cases (WRX, Miata, and even RX7 gen III) look like they will since they appear to have the same mounting provisions, but the geometry of the ends will be off. This would mostly hurt when your suspension goes up and down while cornering, but I have not gone beyond just thinking about it. Terry O (Blueovall Z) or JohnC may have some good thoughts on that.
Davy