Depending on the severity of the crack they can be welded up. However, for long-term success, you need to find someone who is experienced in this problem and has done a lot of them. The correct rod for the welding, and proper preheating and cooling is needed. A shop that handles cars from the 20, 30, 40's established a long time is better than young Willy ( no discredit to Willy) down at the hot rod shop with a cheap stick welder.
Make sure you have the support bracket that goes from the bell housing to the downpipe fitted. That does reduce the vibration and leveraging that the rest of the exhaust system exerts on the manifold.