As far as I know you can put a rifle barrel and stock on a contender frame. T/C used to sell kits for that purpose. Just don't install a pistol barrel on the frame with a rifle stock.So long as it does not have a stock. There is no definition on barrel length with a pistol to my knowledge.
If you use the pistol receiver then it is a pistol. Swapping the 14" barrel onto a receiver marked a rifle would require an SBR Form, but may be classed as an AOW (Any Other Weapon) since it does not fit the defined definition of a rifle due to the lack of a stock.
This is my understanding, you will want to verify this with either the ATF, or a Class III dealer.