The stock factory alternator for a 91 mustang is a 2G... You could use it, but it has issues... Mainly that it does not produce much more amps than the factory Bronco 1G alt and how the battery leads plug into the side. Both of these Alt have external fans.
You want to run a 3G Alt, internal fan. they come in a large case and a small case size. It has been a while, but what I remember is that the large case will fit a Mustang serp set-up. You need to notch (clearance) the Alt bracket slightly so that it will fit. Minor wiring issues for hook up and 100+ amps available for use!
There are lists floating around on Classic of donor cars to get the Alt from... (I remember 94-95 Mustangs) BCB has a wiring diagram on their Tech list