The advantage of the body posts being farther apart is the way the downforce of the body is transferred to the car. I know that the downforce still has to go down through the same parts to get to the chassis, but it is kinda like having a wider wheelbase. Just a little more stability.
You are right that moving them more central should cause less breakage of the shock tower due to less leverage. Go ahead and get two rear shock towers and glue them together.