In my Opinion, pullovers expanding the rib cage is a huge myth. Simply because arnold used them doesn't mean anything (this was a long time ago folks, before scientists joined the game). Your ribs are attached together (ribcage) and are in your body to protect the heart, dumbbells will not cause them to grow bigger..and thats the same for your chest plate. Don't mean to sound so opinionated but until I have some science to back up the facts...
Theres no easy answer bro. Hit the gym, keep a notebook to record your lifting sessions, watch your nutrition, and stay focused..thats the true answer!
Arnold had a massive chest..thanks to genetics..but no enhanced ribcage:
http://www.sfgate.com/c/pictures/200...arzenegger.jpg