With the tires your going to run I would worry about it too much. Unless you show up and run slicks or a DR you will be fine. Also powershifting will eat it up quicker, but I doubt your a power shifter if this is your first Mustang
Look for a Tremec 3550, still a 10 spline input and same 26 spline output on the trans so you can use your stock driveshaft and clutch ( if its an aftermarket unit). They are pretty strong and cheaper than a TKO500/600