If you want just a simple Auto Meter gauge, that can be spliced right into your stock o2 wire off the computer. The only thing with the auto meter a/f ratio gauge is that its not accurate and the lights tend to bounce all over the place. It would be helpful in a very extreme lean or rich condition though.
A wideband a/f monitor on the other hand will tell you your instantaneous a/f ratio. Most have a digital readout that is very easy to understand. They come with a new wideband o2 sensor that replaces for factory one, and most come with a narrowband output that tricks the computer into thinking the stock o2 sensor is still there. The only issue with them is that they cost $300+. If your stang is pretty much stocka wideband isn't a must, however, it is good to have one if you plan to heavily mod in the future.