Well, since you asked... Braces are usually installed to help with load and suspension geometry deflections. Here is how I know for a fact the rear shock tower brace does nothing on a mustang.
1. On a 4 link suspension like you have, all the geometry is determined by the upper and lower control arms. You can reinforce the rear shock towers until you turn blue, it's not going to change anything at all. The suspension links have much stiffer mounting point anyway, and that's called a solid rear axle.
2. There is no load at all on the rear shocks. You can remove the rear shocks completely, and still be able to drive the car around, because all the load is handled by the rear springs, and the control arms. So you can believe there is absolutely no flex at all between those two towers, unless you do coilovers, which something tells me you will not.