It really depends on what you want to do with the car and where you make your power. You can get super nerd-science-research-specific with it, do all the tables with your trans ratios, your tire dimensions, and figure out what final drive is ideal for various scenarios - if you plan on being a serious race car driver or drag racer, then you need to do that.
But, being honest here, if you are just driving it around on the street, then it's not really too much to stress about. My recommendation is something shorter than stock, but not as short as 4.10's. For me, 3.55 is the all around sweet spot. 3.73's are also a consideration. If you really care about MPG (which you probably don't because you are driving an older Mustang
) you could go with 3.08's but I wouldn't bother. Just pick 3.55 or 3.73 and call it a day. I use my Mustang for road racing and lately AutoCrossing. 3.55's are great for road racing (which is what I was originally doing), and are okay for AutoCrossing. If I was ONLY AutoCrossing I'd probably go with the 3.73's and just deal with the higher RPMs on the highway. But the 3.55's serve me well. And they will continue to work well if you mod the engine.