Brian answered you about the shocks. I believe the wording is something about mounting points and ride height remaining unchanged (no adjustable spring perches, for example), but adjustable shocks are generally fine. Double-adjustables are nice, but in practice I don't adjust mine very often. In fact, I would argue that a set of non-adjustable shocks that are custom-valved for a particular application are a better bet than adjustables that you have to tune yourself.
For the tire, it's good advice to see what the fast guys are doing and imitate them, but in your case, none of the fast guys are running street tires on a FS Mustang, and street tires behave much differently than R-comps. I would suggest going with the 245/45, but that's just me. If the 255/40 will give you significantly improved gearing and still allow you to hit upwards of 60mph in 2nd gear, you should consider it. For some reason the 245mm Bridgestones are much, much cheaper than the 255mm version, which makes the decision easier.
Mississippi Region #83 SS / #97 STU