What does it take to use a Remington threaded barrel on a Weatherby Mark V action? Would the threads have to be cut off and barrel re-threaded or can the threads be modified to fit?

Remington barrel is the same thread pitch and diameter as a Weatherby MK V But you would need to head space the barrel and the face of the barrel would need to be faced to work with the Weatherby

If it's a Varmint master the threads would need to be re cut as the diameter is not the same.