Don't do what I did, if your are planing on putting u/d pulleys on do it now. I used a gattor back belt and I think the size was 72 something. I will post it when I get home. One way a friend did it was to have someone hold the tensioner back to where your stock belt is holding the tensioner now. Then route the stock belt to where it is tight against the tensioner, and with some vise grips pinch the belt, that will tell you more or less the size of the belt you need. Good Luck
Hopefully ur stang is driveable right now cuz you will be making several trips to the parts store. Took me three tries b4 i got it right so if u could drive it up there it will save u some time. Also ive heard of guys taking measurement with a tailor style measuring tape but dont count on it being perfect. Laters