First step when comparing tires should always be a tire height calculator.
Your current 255 40 19 is 27" tall
the 245 35 19 is 25.75" tall
Granted, the tread width on the PSS is 9.2", so with tread width being a little wider than your rim you may get a little more height from the sidwewall... maybe like 25.8" instead of 25.75. If it were me i would not. that's an incredibly short tire on a mustang, and aside from your car handling goofy with tires that mis matched front and rear, it will likely just look stupid.