This site is about old school ads..so keeping it short about rib box expansion.
The cartilage expands between the ribs and the center of the chest, not the rib bones themselves. The average of human cartilage becoming fixed or harden is around 24 to 26 years of age. After that not much more expansion is possible. Getting serious about pullovers, with heavy breathing compound exercises like squats have increased men's total chest measurements from 2 to 3 inches..and in a somewhat short period of time........if within that younger age group. Probably thousands of men, past and present, have accomplished this.
Pullover's alone (straight or bent arms) can also give a larger rib box, but more dramatic results is with a heavy breathing combo exercise. The moon and half moon benches where used quite a bit back in the day. Along with the regular flat bench.