To simplify this rifle/pistol thing it's perfectly legal to use a 16" or longer barrel but if a shoulder stock is added then the barrel MUST be 16" or longer. It's perfectly legal to convert that Contender into a carbine by fitting a shoulder stock and 16+" barrel and as long as it was originally sold as a pistol it's perfectly legal to switch back and forth, just don't use a short barrel with the shoulder stock! A rifle however CAN NOT be legally converted to a pistol! As long as it started as pistol it can go either way but if it starts as a rifle it must remain in a rifle configuration.
So to make a long story short if you want a 16+" barrel go for it, you will be legal either way and with that barrel you can also opt for the carbine configuration.